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.

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

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;
    }
}