First Unique Character in a String Problem & Solution
Given a string s, find the first non-repeating character in it and return its index.
If it does not exist, return -1.
See the first unique character in a string problem on LeetCode.
C++ Solution
The runtime complexity of this algorithm is $O(n)$.
#pragma GCC optimize("Ofast")
#pragma GCC optimization("unroll-loops")
static const int _=[](){ios::sync_with_stdio(false);cin.tie(nullptr);cout.tie(nullptr);return 0;}();
class Solution {
public:
int firstUniqChar(string s) {
vector<int> positions('z' - 'a' + 1, -1);
vector<bool> repeats('z' - 'a' + 1, false);
for (int i = 0; i < s.size(); ++i) {
int offset = s[i] - 'a';
if (positions[offset] >= 0) {
repeats[offset] = true;
}
if (!repeats[offset]) {
positions[offset] = i;
}
}
int minarg = INT_MAX;
for (int i = 0; i < repeats.size(); ++i) {
if (!repeats[i] && positions[i] >= 0) {
minarg = min(minarg, positions[i]);
}
}
return minarg == INT_MAX ? -1 : minarg;
}
};