617, Merge Two Binary Trees

I Problem

You are given two binary trees root1 and root2.

Imagine that when you put one of them to cover the other, some nodes of the two trees are overlapped while the others are not. You need to merge the two trees into a new binary tree. The merge rule is that if two nodes overlap, then sum node values up as the new value of the merged node. Otherwise, the NOT null node will be used as the node of the new tree.

Return the merged tree.

Note: The merging process must start from the root nodes of both trees.

Example 1

Input: root1 = [1, 3, 2, 5], root2 = [2, 1, 3, null, 4, null, 7]
Output: [3, 4, 5, 5, 4, null, 7]

Example 2
Input: root1 = [1], root2 = [1, 2]
Output: [2, 2]

Constraints

• The number of nodes in both trees is in the range [0, 2000]
• -10⁴ <= Node.val <= 10⁴

Related Topics

• Tree
• Depth-First Search
• Breadth-First Search
• Binary Tree

II Solution

#[derive(Debug, PartialEq, Eq)]
pub struct TreeNode {
    pub val: i32,
    pub left: Option<Rc<RefCell<TreeNode>>>,
    pub right: Option<Rc<RefCell<TreeNode>>>,
}

impl TreeNode {
    #[inline]
    pub fn new(val: i32) -> Self {
        TreeNode {
            val,
            left: None,
            right: None,
        }
    }
}

public class TreeNode {
    int val;
    TreeNode left;
    TreeNode right;

    TreeNode() {}
    TreeNode(int val) { this.val = val; }
    TreeNode(int val, TreeNode left, TreeNode right) {
        this.val = val;
        this.left = left;
        this.right = right;
    }
}

Approach 1: Depth-First Search

pub fn merge_trees(root1: Option<Rc<RefCell<TreeNode>>>, root2: Option<Rc<RefCell<TreeNode>>>) -> Option<Rc<RefCell<TreeNode>>> {
    //Self::dfs_recur_create_new(root1, root2)
    //Self::dfs_iter_create_new(root1, root2)
    //Self::dfs_recur_reuse(root1, root2)
    Self::dfs_iter_reuse(root1, root2)
}

///
/// DFS, recursion, create a new node
///
fn dfs_recur_create_new(
    root1: Option<Rc<RefCell<TreeNode>>>,
    root2: Option<Rc<RefCell<TreeNode>>>,
) -> Option<Rc<RefCell<TreeNode>>> {
    const MERGE: fn(
        Option<Rc<RefCell<TreeNode>>>,
        Option<Rc<RefCell<TreeNode>>>,
    ) -> Option<Rc<RefCell<TreeNode>>> = |root1, root2| match (root1, root2) {
        (None, r2) => r2,
        (r1, None) => r1,
        (Some(r1), Some(r2)) => {
            let mut r1 = r1.borrow_mut();
            let mut r2 = r2.borrow_mut();
            let root = Rc::new(RefCell::new(TreeNode::new(r1.val + r2.val)));

            root.borrow_mut().left = MERGE(r1.left.take(), r2.left.take());
            root.borrow_mut().right = MERGE(r1.right.take(), r2.right.take());

            Some(root)
        }
    };

    MERGE(root1, root2)
}

///
/// DFS, iteration, create a new node
///
fn dfs_iter_create_new(
    root1: Option<Rc<RefCell<TreeNode>>>,
    root2: Option<Rc<RefCell<TreeNode>>>,
) -> Option<Rc<RefCell<TreeNode>>> {
    let mut root = None;
    let mut stack = vec![(None, root1, root2, false)];

    while let Some((parent, r1, r2, is_left)) = stack.pop() {
        let node = match (r1, r2) {
            (None, r2) => r2,
            (r1, None) => r1,
            (Some(r1), Some(r2)) => {
                let mut r1 = r1.borrow_mut();
                let mut r2 = r2.borrow_mut();
                let node = Some(Rc::new(RefCell::new(TreeNode::new(r1.val + r2.val))));

                if r1.right.is_some() || r2.right.is_some() {
                    stack.push((node.clone(), r1.right.take(), r2.right.take(), false));
                }
                if r1.left.is_some() || r2.left.is_some() {
                    stack.push((node.clone(), r1.left.take(), r2.left.take(), true));
                }

                node
            }
        };

        if let Some(p) = parent {
            if is_left {
                p.borrow_mut().left = node;
            } else {
                p.borrow_mut().right = node;
            }
        } else {
            root = node;
        }
    }

    root
}

///
/// DFS, recursion, reuse root1
///
fn dfs_recur_reuse(
    root1: Option<Rc<RefCell<TreeNode>>>,
    root2: Option<Rc<RefCell<TreeNode>>>,
) -> Option<Rc<RefCell<TreeNode>>> {
    const MERGE: fn(
        Option<Rc<RefCell<TreeNode>>>,
        Option<Rc<RefCell<TreeNode>>>,
    ) -> Option<Rc<RefCell<TreeNode>>> = |root1, root2| match (root1, root2) {
        (None, r2) => r2,
        (r1, None) => r1,
        (Some(r1), Some(r2)) => {
            r1.borrow_mut().val += r2.borrow().val;
            let r1_l = r1.borrow_mut().left.take();
            let r2_l = r2.borrow_mut().left.take();
            let r1_r = r1.borrow_mut().right.take();
            let r2_r = r2.borrow_mut().right.take();

            r1.borrow_mut().left = MERGE(r1_l, r2_l);
            r1.borrow_mut().right = MERGE(r1_r, r2_r);

            Some(r1)
        }
    };

    MERGE(root1, root2)
}

///
/// DFS, iteration, reuse root1
///
fn dfs_iter_reuse(
    root1: Option<Rc<RefCell<TreeNode>>>,
    root2: Option<Rc<RefCell<TreeNode>>>,
) -> Option<Rc<RefCell<TreeNode>>> {
    // Ensure that r1 is not None
    if root1.is_none() {
        return root2;
    }
    let mut stack = vec![(root1.clone(), root2)];

    while let Some((root1, root2)) = stack.pop() {
        match (root1, root2) {
            (Some(r1), Some(r2)) => {
                let mut r1 = r1.borrow_mut();
                let mut r2 = r2.borrow_mut();

                r1.val += r2.val;
                if r1.left.is_none() {
                    r1.left = r2.left.take();
                } else {
                    stack.push((r1.left.clone(), r2.left.clone()));
                }
                if r1.right.is_none() {
                    r1.right = r2.right.take();
                } else {
                    stack.push((r1.right.clone(), r2.right.clone()));
                }
            }
            _ => {}
        }
    }

    root1
}

public TreeNode mergeTrees(TreeNode root1, TreeNode root2) {
    //return this.dfsRecurCreateNew(root1, root2);
    //return this.dfsIterCreateNew(root1, root2);
    //return this.dfsRecurReuse(root1, root2);
    return this.dfsIterReuse(root1, root2);
}

BiFunction<TreeNode, TreeNode, TreeNode> merge1 = (root1, root2) -> {
    if (root1 == null) {
        return root2;
    }
    if (root2 == null) {
        return root1;
    }

    TreeNode root = new TreeNode(root1.val + root2.val);
    root.left = this.merge1.apply(root1.left, root2.left);
    root.right = this.merge1.apply(root1.right, root2.right);

    return root;
};
/**
 * DFS, recursion, create a new node
 */
TreeNode dfsRecurCreateNew(TreeNode root1, TreeNode root2) {
    return this.merge1.apply(root1, root2);
}

@FunctionalInterface
interface TriFunction<A, B, C, D> {
    D apply(A a, B b, C c);
}
TriFunction<TreeNode, TreeNode, Deque<Object[]>, TreeNode> createNode = (r1, r2, container) -> {
    if (r1 == null) {
        return r2;
    }
    if (r2 == null) {
        return r1;
    }

    TreeNode node = new TreeNode(r1.val + r2.val);
    if (r1.left != null || r2.left != null) {
        container.addLast(new Object[]{node, r1.left, r2.left, true});
    }
    if (r1.right != null || r2.right != null) {
        container.addLast(new Object[]{node, r1.right, r2.right, false});
    }

    return node;
};
/**
 * DFS, iteration, create a new node
 */
TreeNode dfsIterCreateNew(TreeNode root1, TreeNode root2) {
    TreeNode root = null;
    Deque<Object[]> stack = new ArrayDeque<>() {{
        this.push(new Object[] {
            null, root1, root2, false
        });
    }};

    while (!stack.isEmpty()) {
        Object[] objs = stack.pop();
        TreeNode parent = (TreeNode) objs[0];
        TreeNode r1 = (TreeNode) objs[1];
        TreeNode r2 = (TreeNode) objs[2];
        boolean isLeft = (boolean) objs[3];

        TreeNode node = this.createNode.apply(r1, r2, stack);

        if (parent == null) {
            root = node;
        } else {
            if (isLeft) {
                parent.left = node;
            } else {
                parent.right = node;
            }
        }
    }

    return root;
}

BiFunction<TreeNode, TreeNode, TreeNode> merge2 = (root1, root2) -> {
    if (root1 == null) {
        return root2;
    }
    if (root2 == null) {
        return root1;
    }

    root1.val += root2.val;
    root1.left = this.merge2.apply(root1.left, root2.left);
    root1.right = this.merge2.apply(root1.right, root2.right);

    return root1;
};
/**
 * DFS, recursion, reuse root1
 */
TreeNode dfsRecurReuse(TreeNode root1, TreeNode root2) {
    return this.merge2.apply(root1, root2);
}

/**
 * DFS, iteration, reuse root1
 */
TreeNode dfsIterReuse(TreeNode root1, TreeNode root2) {
    // 确保root1不为null
    if (root1 == null) {
        return root2;
    }
    Deque<TreeNode[]> stack = new ArrayDeque<>() {{
        this.push(new TreeNode[]{root1, root2});
    }};

    while (!stack.isEmpty()) {
        TreeNode[] nodes = stack.pop();
        TreeNode r1 = nodes[0];
        TreeNode r2 = nodes[1];
        if (r1 == null || r2 == null) {
            continue;
        }

        r1.val += r2.val;
        if (r1.left == null) {
            r1.left = r2.left;
        } else {
            stack.push(new TreeNode[]{r1.left, r2.left});
        }
        if (r1.right == null) {
            r1.right = r2.right;
        } else {
            stack.push(new TreeNode[]{r1.right, r2.right});
        }
    }

    return root1;
}

Approach 2: Breadth-First Search

pub fn merge_trees(root1: Option<Rc<RefCell<TreeNode>>>, root2: Option<Rc<RefCell<TreeNode>>>) -> Option<Rc<RefCell<TreeNode>>> {
    //Self::bfs_iter_create_new(root1, root2)
    Self::bfs_iter_reuse(root1, root2)
}

///
/// BFS, iteration, create a new node
///
fn bfs_iter_create_new(
    root1: Option<Rc<RefCell<TreeNode>>>,
    root2: Option<Rc<RefCell<TreeNode>>>,
) -> Option<Rc<RefCell<TreeNode>>> {
    let mut root = None;
    let mut queue = VecDeque::from([(None, root1, root2, false)]);

    while let Some((parent, r1, r2, is_left)) = queue.pop_front() {
        let node = match (r1, r2) {
            (None, r2) => r2,
            (r1, None) => r1,
            (Some(r1), Some(r2)) => {
                let mut r1 = r1.borrow_mut();
                let mut r2 = r2.borrow_mut();
                let new = Some(Rc::new(RefCell::new(TreeNode::new(r1.val + r2.val))));

                if r1.left.is_some() || r2.left.is_some() {
                    queue.push_back((new.clone(), r1.left.take(), r2.left.take(), true));
                }
                if r1.right.is_some() || r2.right.is_some() {
                    queue.push_back((new.clone(), r1.right.take(), r2.right.take(), false));
                }

                new
            }
        };

        if let Some(p) = parent {
            if is_left {
                p.borrow_mut().left = node;
            } else {
                p.borrow_mut().right = node;
            }
        } else {
            root = node;
        }
    }

    root
}

///
/// BFS, iteration, reuse root1
///
fn bfs_iter_reuse(
    root1: Option<Rc<RefCell<TreeNode>>>,
    root2: Option<Rc<RefCell<TreeNode>>>,
) -> Option<Rc<RefCell<TreeNode>>> {
    // Ensure that r1 is not None
    if root1.is_none() {
        return root2;
    }
    let mut queue = VecDeque::from([(root1.clone(), root2)]);

    while let Some((r1, r2)) = queue.pop_front() {
        match (r1, r2) {
            (Some(r1), Some(r2)) => {
                let mut r1 = r1.borrow_mut();
                let mut r2 = r2.borrow_mut();

                r1.val += r2.val;
                if r1.left.is_none() {
                    r1.left = r2.left.take();
                } else {
                    queue.push_back((r1.left.clone(), r2.left.clone()));
                }
                if r1.right.is_none() {
                    r1.right = r2.right.take();
                } else {
                    queue.push_back((r1.right.clone(), r2.right.clone()));
                }
            }
            _ => {}
        }
    }

    root1
}

public TreeNode mergeTrees(TreeNode root1, TreeNode root2) {
    //return this.bfsIterCreateNew(root1, root2);
    return this.bfsIterReuse(root1, root2);
}

/**
 * BFS, iteration, create a new node
 */
TreeNode bfsIterCreateNew(TreeNode root1, TreeNode root2) {
    TreeNode root = null;
    Deque<Object[]> queue = new ArrayDeque<>() {{
        this.addLast(new Object[]{null, root1, root2, false});
    }};

    while (!queue.isEmpty()) {
        Object[] objs = queue.removeFirst();
        TreeNode parent = (TreeNode) objs[0];
        TreeNode r1 = (TreeNode) objs[1];
        TreeNode r2 = (TreeNode) objs[2];
        boolean isLeft = (boolean) objs[3];

        TreeNode node = this.createNode.apply(r1, r2, queue);

        if (parent == null) {
            root = node;
        } else {
            if (isLeft) {
                parent.left = node;
            } else {
                parent.right = node;
            }
        }
    }

    return root;
}

/**
 * BFS, iteration, reuse root1
 */
TreeNode bfsIterReuse(TreeNode root1, TreeNode root2) {
    if (root1 == null) {
        return root2;
    }
    Deque<TreeNode[]> queue = new ArrayDeque<>() {{
        this.addLast(new TreeNode[]{root1, root2});
    }};

    while (!queue.isEmpty()) {
        TreeNode[] nodes = queue.removeFirst();
        TreeNode r1 = nodes[0];
        TreeNode r2 = nodes[1];

        if (r1 == null || r2 == null) {
            continue;
        }
        r1.val += r2.val;

        if (r1.left == null) {
            r1.left = r2.left;
        } else {
            queue.addLast(new TreeNode[]{r1.left, r2.left});
        }
        if (r1.right == null) {
            r1.right = r2.right;
        } else {
            queue.addLast(new TreeNode[]{r1.right, r2.right});
        }
    }

    return root1;
}