# [859. Buddy Strings](https://leetcode.com/problems/buddy-strings/)

## 题目

Given two strings s and goal, return true if you can swap two letters in s so the result is equal to goal, otherwise, return false.

Swapping letters is defined as taking two indices i and j (0-indexed) such that i != j and swapping the characters at s[i] and s[j].

For example, swapping at indices 0 and 2 in "abcd" results in "cbad".

**Example 1**:

    Input: s = "ab", goal = "ba"
    Output: true
    Explanation: You can swap s[0] = 'a' and s[1] = 'b' to get "ba", which is equal to goal.

**Example 2**:

    Input: s = "ab", goal = "ab"
    Output: false
    Explanation: The only letters you can swap are s[0] = 'a' and s[1] = 'b', which results in "ba" != goal.

**Example 3**:

    Input: s = "aa", goal = "aa"
    Output: true
    Explanation: You can swap s[0] = 'a' and s[1] = 'a' to get "aa", which is equal to goal.

**Example 4**:

    Input: s = "aaaaaaabc", goal = "aaaaaaacb"
    Output: true

**Constraints:**

- 1 <= s.length, goal.length <= 2 * 10000
- s and goal consist of lowercase letters.

## 题目大意

给你两个字符串 s 和 goal ，只要我们可以通过交换 s 中的两个字母得到与 goal 相等的结果，就返回 true；否则返回 false 。

交换字母的定义是：取两个下标 i 和 j （下标从 0 开始）且满足 i != j ，接着交换 s[i] 和 s[j] 处的字符。

例如，在 "abcd" 中交换下标 0 和下标 2 的元素可以生成 "cbad" 。

## 解题思路

分为两种情况进行比较:
- s 等于 goal, s 中有重复元素就返回 true,否则返回 false
- s 不等于 goal, s 中有两个下标不同的字符与 goal 中对应下标的字符分别相等

## 代码

```go

package leetcode

func buddyStrings(s string, goal string) bool {
	if len(s) != len(goal) || len(s) <= 1 {
		return false
	}
	mp := make(map[byte]int)
	if s == goal {
		for i := 0; i < len(s); i++ {
			if _, ok := mp[s[i]]; ok {
				return true
			}
			mp[s[i]]++
		}
		return false
	}
	first, second := -1, -1
	for i := 0; i < len(s); i++ {
		if s[i] != goal[i] {
			if first == -1 {
				first = i
			} else if second == -1 {
				second = i
			} else {
				return false
			}
		}
	}
	return second != -1 && s[first] == goal[second] && s[second] == goal[first]
}
```