541. Reverse String II

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]

C++(19ms):

class Solution {
public:
    string reverseStr(string s, int k) {
        string res = "" ;
        string t ;
        int left = 0 ;
        int m = 1 ;
        while(left < s.size()){
            t = s.substr(left,k) ;
            if(m%2){
               reverse(t.begin(),t.end()) ;
            }
            m++;
            left+=k ;
            res+=t ;
        }
        return res ;
    }
};

C++(9ms):

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