# 387.First-Unique-Character-in-a-String

## 387. First Unique Character in a String

## 题目地址

<https://leetcode.com/problems/first-unique-character-in-a-string/>

## 题目描述

```
Given a string, find the first non-repeating character in it and return it's index. If it doesn't exist, return -1.

Examples:
s = "leetcode"
return 0.

s = "loveleetcode",
return 2.
Note: You may assume the string contain only lowercase letters.
```

## 代码

### Approach #1  Linear time solution

**Complexity Analysis**

* Time complexity : O(*N*) since we go through the string of length `N` two times.
* Space complexity : O(*N*) since we have to keep a hash map with `N` elements.

```java
class Solution {
  public int firstUniqChar(String s) {
        HashMap<Character, Integer> count = new HashMap<Character, Integer>();
    int n = s.length();
    for (int i = 0; i < n; i++) {
      char c = s.charAt(i);
      count.put(c, count.getOrDefault(c, 0) + 1);
    }

    for (int i = 0; i < n; i++) {
      if (count.get(s.charAt(i)) == 1) {
        return i;
      }
    }

    return -1;
  }
}
```

### #2

```java
public class Solution {
    public int firstUniqChar(String s) {
        int count [] = new int[26];
        for (int i = 0; i < s.length(); i ++) {
          count[s.charAt(i) - 'a'] ++;
        }

        for (int i = 0; i < s.length(); i ++) {
           if (count[s.charAt(i) - 'a'] == 1) {
              return i;
            }
        } 
        return -1;
    }
}
```