541. Reverse String II

Problem:

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:

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

思路：

Solution (C++):

string reverseStr(string s, int k) {
    int n = s.length();
    for (int i = 0; i < n; i += 2*k) {
        if (i + k < n) {
            for (int j = 0; j < k/2; ++j) {
                swap(s[i+j], s[i+k-1-j]);
            }                
        } else {
            for (int j = 0; j < (n-i)/2; ++j) {
                swap(s[i+j], s[n-1-j]);
            }
        }
    }
    return s;
}

性能：

Runtime: 4 ms  Memory Usage: 7.2 MB

思路：

Solution (C++):

性能：

Runtime: ms  Memory Usage: MB