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.
这道题确实没有什么难度，我们只要用哈希表建立每个字符和其出现次数的映射，然后按顺序遍历字符串，找到第一个出现次数为1的字符，返回其位置即可，参见代码如下：
class Solution {
public:
int firstUniqChar(string s) {
unordered_map<char, int> m;
for (char c : s) ++m[c];
for (int i = ; i < s.size(); ++i) {
if (m[s[i]] == ) return i;
}
return -;
}
};
类似题目：
参考资料：
https://leetcode.com/problems/first-unique-character-in-a-string/