Minimum Swaps to Make Strings Equal

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'.

class Solution {
    public int minimumSwap(String s1, String s2) {
        if (s1.length() != s2.length())
            return -1;
        int x1 = 0; // number of 'x' in s1 (skip equal chars at same index)
        int y1 = 0; // number of 'y' in s1 (skip equal chars at same index)
        int x2 = 0; // number of 'x' in s2 (skip equal chars at same index)
        int y2 = 0; // number of 'y' in s2 (skip equal chars at same index)

        for (int i = 0; i < s1.length(); i++) {
            char c1 = s1.charAt(i);
            char c2 = s2.charAt(i);
            if (c1 == c2) // skip chars that are equal at the same index in s1 and s2
                continue;
            if (c1 == 'x')
                x1++;
            else
                y1++;
            if (c2 == 'x')
                x2++;
            else
                y2++;
        }

        // After skip "c1 == c2", check the number of 'x' and 'y' left in s1 and s2.
        if ((x1 + x2) % 2 != 0 || (y1 + y2) % 2 != 0)
            return -1; // if number of 'x' or 'y' is odd, we can not make s1 equals to s2

        int swaps = x1 / 2 + y1 / 2 + (x1 % 2) * 2;
        // Cases to do 1 swap:
        // "xx" => x1 / 2 => how many pairs of 'x' we have ?
        // "yy" => y1 / 2 => how many pairs of 'y' we have ?
        //
        // Cases to do 2 swaps:
        // "xy" or "yx" => x1 % 2

        return swaps;
    }
}