LeetCode Problem 3.Longest Substring Without Repeating Characters

Given a string, find the length of the longest substring without repeating characters.

Examples:

Given "abcabcbb", the answer is "abc", which the length is 3.

Given "bbbbb", the answer is "b", with the length of 1.

Given "pwwkew", the answer is "wke", with the length of 3. Note that the answer must be a substring, "pwke" is a subsequence and not a substring.

这题如果想知道是否有重复字符，则扫描每个字母时必定需要有记录操作。所有字符可以放在一个数组里，以字符作为下标存储它们最后一次出现的位置。如果某个字符已经有记录了，而且这个字符的位置在子串左边界的右边（含左边界），说明字符重复了，那么子串的左边界要重新赋值为重复字符的位置的下一个位置，用于把最左边的重复字符去掉。每扫描一个字母就更新一下子串的最长长度。

代码如下：

 1 class Solution {
 2 public:
 3     int lengthOfLongestSubstring(string s) {
 4         int max_size(0), left_bound(0), str_size(s.length());
 5         int last_positions[256];
 6         memset(last_positions, -1, sizeof(last_positions));
 7
 8         for (int i = 0; i < str_size; ++i){
 9             if (last_positions[s[i]] >= left_bound){
10                 left_bound = last_positions[s[i]] + 1;
11             }
12             last_positions[s[i]] = i;
13             max_size = max(max_size, i - left_bound + 1);
14         }
15         return max_size;
16     }
17 };