[LeetCode] Algorithms-3.Longest Substring Without Repeating Characters

描述

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.

思路

最开始做这道题的时候，想用暴力解，时间复杂度为O(N^2)，但是会超时，没办法，只有想一想去他的办法，后来想到了一个思路，因为要求的子字符串是不能有重复字符的，那么在遍历字符串的时候，当遇到和之前重复的字符时，就是一个可能的最大值，所以最后的大概想法是每次检查第 i 个字符上一次出现的位置，当前位置和上一次出现的位置的差值就是当前字符结尾无重复子串最大长度，再跟所有无重复子串最大长度比较

代码

class Solution {
public:
    int lengthOfLongestSubstring(string s) {
        int n = 0, table[256], left = -1;
        memset(table, -1, 256 * sizeof(int));
        for(int i = 0; i < s.length(); i++) {
            if(table[s[i]] > left) {
                left = table[s[i]];
            }
            n = max(n, i - left);
            table[s[i]] = i;
        }
        return n;
    }
};

class Solution {
public:
    int lengthOfLongestSubstring(string s) {
        map<char,int> maps;
        int i,Max=0,pre=-1;
        for(i=0;i<s.length();i++) maps[s[i]]=-1;
        for(i=0;i<s.length();i++)
        {
            pre=max(pre,maps[s[i]]);
            Max=max(Max,i-pre);
            maps[s[i]]=i;
        }
        return Max;
    }
};