leetcode/java/387_First_Unique_Character_in_a_String.java at master · cphysics/leetcode

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

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

Count frequencies of each letter. Iterate over string again, return index of first letter with frequency of 1.

Time - O(n)

Space - O(1)

public class Solution {

public int firstUniqChar(String s) {

int[] frequencies = new int[26];

for (int i = 0; i < s.length(); ++i)

++frequencies[(int) s.charAt(i) - (int) 'a'];

for (int i = 0; i < s.length(); ++i)

if (frequencies[(int) s.charAt(i) - (int) 'a'] == 1)

return i;

return -1;

}