387. First Unique Character in a String
Tags: ‘Hash Table’, ‘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.
Solution
方法1： 线性扫描，Naive $O(n) time O(1) space$
public int firstUniqChar(String s) {
Map<Character, Integer> map = new HashMap<>();
for (int i = 0; i < s.length(); i++) {
char c = s.charAt(i);
if (!map.containsKey(c)) {
map.put(c, i); // mark index
} else {
map.put(c, -1);// repeat, mark as -1
}
}
// find the smallest index >= 0
int min = Integer.MAX_VALUE;
for (Integer x: map.values()) {
if (x >= 0 && x < min) {
min = x;
}
}
return min == Integer.MAX_VALUE ? -1 : min;
}