You are given two strings ​​s1​​​ and ​​s2​​​ of equal length consisting of letters ​​"x"​​​ and ​​"y"​​ only. Your task is to make these two strings equal to each other. You can swap any two characters that belong to different strings, which means: swap ​​s1[i]​​​ and ​​s2[j]​​.

Return the minimum number of swaps required to make ​​s1​​​ and ​​s2​​​ equal, or return ​​-1​​ if it is impossible to do so.

 

Example 1:

Input: s1 = "xx", s2 = "yy"
Output: 1
Explanation: 
Swap s1[0] and s2[1], s1 = "yx", s2 = "yx".

Example 2: 

Input: s1 = "xy", s2 = "yx"
Output: 2
Explanation: 
Swap s1[0] and s2[0], s1 = "yy", s2 = "xx".
Swap s1[0] and s2[1], s1 = "xy", s2 = "xy".
Note that you can't swap s1[0] and s1[1] to make s1 equal to "yx", cause we can only swap chars in different strings.

Example 3:

Input: s1 = "xx", s2 = "xy"
Output: -1

Example 4:

Input: s1 = "xxyyxyxyxx", s2 = "xyyxyxxxyx"
Output: 4

 

Constraints:

  • ​​1 <= s1.length, s2.length <= 1000​​
  • ​​s1, s2​​​ only contain​​'x'​​​ or​​'y'​​.

题解：

不会，看了解答也想不通那种，可能我脑子有问题吧。

class Solution {
public:
    int minimumSwap(string s1, string s2) {
        int l1 = s1.length(), l2 = s2.length();
        if (l1 != l2) {
            return -1;
        }
        int xy = 0, yx = 0;
        for (int i = 0; i < l1; i++) {
            if (s1[i] != s2[i]) {
                if (s1[i] == 'x') {
                    xy++;
                }
                else {
                    yx++;
                }
            }
        }
        if ((xy + yx) % 2 != 0) {
            return -1;
        }
        return xy / 2 + yx / 2 + (xy % 2) * 2;
    }
};