387. 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.

Examples:

s = "leetcode"
return 0.

s = "loveleetcode",
return 2.

Note: You may assume the string contain only lowercase letters.

用一個size為26的record array紀錄每個字母出現的次數，然後在掃描input string一遍，一發現有出現次數為1的字元的index就是答案。

// Two Pass Method
int firstUniqChar(string s) { // time: O(n); space: O(1)
    vector<int> record(26, 0);
    for (char ch : s) ++record[ch - 'a'];
    for (int i = 0; i < s.length(); ++i) {
        if (record[s[i] - 'a'] == 1) return i;
    }
    return -1;
}