541, Reverse String II

I Problem

Given a string s and an integer k, reverse the first k characters for every 2k characters counting from the start of the string.

• If there are fewer than k characters left, reverse all of them.
• If there are less than 2k but greater than or equal to k characters, then reverse the first k characters and leave the other as original.

Example 1
Input: s = "abcdefg", k = 2
Output: "bacdfeg"

Example 2
Input: s = "abcd", k = 2
Output: "bacd"

Constraints

• 1 <= s.length <= 10⁴
• s consists of only lowercase English letters
• 1 <= k <= 10⁴

Related Topics

• Two Pointers
• String

II Solution

Approach 1: Two Pointers

Rust

pub fn reverse_str(s: String, k: i32) -> String {
    let k = k as usize;
    let len = s.len();
    let p = unsafe { s.as_bytes_mut() };
    let mut begin = 0_usize;
    let mut end = begin + k;

    loop {
        if begin >= len {
            break;
        }
        if end > len {
            end = len;
        }

        let mut l = begin;
        let mut r = end - 1;
        while l < r {
            p.swap(l, r);
            l += 1;
            r -= 1;
        }

        begin += 2 * k;
        end = begin + k;
    }

    s
}

Java

public String reverseStr(String s, int k) {
    char[] chars = s.toCharArray();
    int len = chars.length;
    int begin = 0;
    int end = begin + k;

    do {
        if (end > len) {
            end = len;
        }

        int l = begin;
        int r = end - 1;
        while (l < r) {
            char temp = chars[l];
            chars[l] = chars[r];
            chars[r] = temp;
            l++;
            r--;
        }

        begin += 2 * k;
        end = begin + k;
    } while (begin < len);

    return String.valueOf(chars);
}