389. Find the Difference

You are given two strings `s` and `t`.

String `t` is generated by random shuffling string `s` and then add one more letter at a random position.

Return the letter that was added to `t`.

Example 1:

```Input: s = "abcd", t = "abcde"
Output: "e"
Explanation: 'e' is the letter that was added.
```

Example 2:

```Input: s = "", t = "y"
Output: "y"
```

Example 3:

```Input: s = "a", t = "aa"
Output: "a"
```

Example 4:

```Input: s = "ae", t = "aea"
Output: "a"
```

Constraints:

• `0 <= s.length <= 1000`
• `t.length == s.length + 1`
• `s` and `t` consist of lower-case English letters.

389. Find the Difference
``````struct Solution;

use std::collections::HashMap;

impl Solution {
fn find_the_difference(s: String, t: String) -> char {
let mut hm: HashMap<char, i32> = HashMap::new();
for c in t.chars() {
let e = hm.entry(c).or_default();
*e += 1;
}
for c in s.chars() {
let e = hm.entry(c).or_default();
*e -= 1;
}
for (&c, &v) in hm.iter() {
if v == 1 {
return c;
}
}
unreachable!()
}
}

#[test]
fn test() {
let s = "abcd".to_string();
let t = "abcde".to_string();
let c = 'e';
assert_eq!(Solution::find_the_difference(s, t), c);
}
``````