public class Solution
extends Object
1247 - Minimum Swaps to Make Strings Equal\.
Medium
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 cannot 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
**Constraints:**
* `1 <= s1.length, s2.length <= 1000`
* `s1, s2` only contain `'x'` or `'y'`.