LeetCode 3. Longest Substring Without Repeating Characters

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

Example 1:

Input: "abcabcbb"
Output: 3 
Explanation: The answer is "abc", with the length of 3. 

Example 2:

Input: "bbbbb"
Output: 1
Explanation: The answer is "b", with the length of 1.

Example 3:

Input: "pwwkew"
Output: 3
Explanation: 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.

给一个字符串，让我们求最长的无重复字符的子串得长度。

思路：遍历字符串，将字符存在HashMap中，建立每个字符和其最后出现位置之间的映射，然后我们定义两个变量res和left，其中res用来记录最长无重复子串的长度，left指向该无重复子串左边的起始位置的前一个，由于是前一个，所以初始化就是-1，然后我们遍历整个字符串，对于每一个遍历到的字符，如果该字符已经在HashMap中存在了，并且如果其映射值大于left的话，那么更新left为当前映射值。然后映射值更新为当前坐标i，这样保证了left始终为当前边界的前一个位置，然后计算长度的时候，直接用i-left即可，用来更新结果res，通过max即时选择res极大值即可，遍历结束获得res最大值。

//C++
class Solution {
public:
    int lengthOfLongestSubstring(string s) {
        int res = 0, left = -1, n = s.size();
        unordered_map<int, int> m;
        for (int i = 0; i < n; i++) {
            if (m.count(s[i]) && m[s[i]] > left) {
                left = m[s[i]];  
            }
            m[s[i]] = i;
            res = max(res, i - left);            
        }
        return res;
    }
};