541. Reverse String II (Easy)

Given a string and an integer k, you need to reverse the first k characters for every 2k characters counting from the start of the string. If there are less 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 left the other as original.

Example:

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

Restrictions:

1. The string consists of lower English letters only.
2. Length of the given string and k will in the range [1, 10000]

Solution:

class Solution {
public:
    string reverseStr(string s, int k) {
        int n = s.size();
        for (int i = 0; i < n; i += 2*k) {
            reverse(s.begin()+i, s.begin()+min(i+k,n));
        }
        return s;
    }
};