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/**

* [50-I] 第一个只出现一次的字符

*

* 题目: 在字符串 s 中找出第一个只出现一次的字符. 如果没有, 返回一个单空格.

*

* 思路: 遍历字符串, 使用哈希表或数组记录字符串中每个字符的出现次数, 然后再次遍历字符串, 当当前字符出现次数为 1 时, 返回即可.

*/

class Solution {

/**

* 时间复杂度: O(n)

* 空间复杂度: O(1)

*/

public char firstUniqChar(String s) {

if (s == null || s.length() == 0) {

return ' ';

}

// use array to store char appear times.

int[] freq = new int[128];

// first traverse for statistics char appear times.

for (char chr : s.toCharArray()) {

freq[chr]++;

}

// second traverse for select the first appear ont time char.

for (char chr : s.toCharArray()) {

if (freq[chr] == 1) {

return chr;

}

}

return ' ';

}

}