3. Longest Substring Without Repeating Characters----LeetCode

3. Longest Substring Without Repeating Characters

Given a string, find the length of the longest substring without repeating characters. For example, the longest substring without repeating letters for "abcabcbb" is "abc", which the length is 3. For "bbbbb" the longest substring is "b", with the length of 1.

題目翻譯：給定一個字符串，找出最長的沒有重複字符的子字符串，輸出該字串長度。例如

“abcabcbb”    最長不重複字串是”abc“     返回長度： 3

”bbbbb“         最長不重複字串是”b“         返回長度： 1

解題思路：利用hash表

class Solution {
public:
    int lengthOfLongestSubstring(string s) {
        int maxlen=0;
        int n=s.length();
        int *next = new int[n];
        int *first = new int[n+1];
        first[n]=n;
        int hash[256];
        memset(hash,n,sizeof(hash));
        for(int i=n-1;i>=0;i--){
            next[i]=hash[s[i]];
            hash[s[i]]=i;
            if(next[i]<first[i+1])
                first[i]=next[i];
            else
                first[i]=first[i+1];
        }
        for(int i=0;i<n;i++){
            if(first[i]-i>maxlen)
                maxlen=first[i]-i;
        }
        delete[]next;
        delete[]first;
        return maxlen;
    }
};

還有一種簡單一些的解法，參考點擊打開鏈接

int lengthOfLongestSubstring(string s) {
        vector<int> dict(256, -1);
        int maxLen = 0, start = -1;
        for (int i = 0; i != s.length(); i++) {
            if (dict[s[i]] > start)
                start = dict[s[i]];
            dict[s[i]] = i;
            maxLen = max(maxLen, i - start);
        }
        return maxLen;
    }