C++版- Leetcode 3. Longest Substring Without Repeating Characters解题报告

Leetcode 3. Longest Substring Without Repeating Characters

 

提交网址： https://leetcode.com/problems/longest-substring-without-repeating-characters/

Total Accepted: 149135 Total Submissions: 674678 Difficulty: Medium

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.

分析：

使用哈希表，保存每个字符上一次出现的位置，时间复杂度为O(n).

AC代码：

class Solution {
public:
    int lengthOfLongestSubstring(string s) {
        int loc[256];     // 哈希表保存字符上一次出现的位置，ASCII码是8位，一般用前7位，是128(2^7)个可用字符。IBM机器的扩展使用了最高位，故用2^8个(256)
        memset(loc, -1, sizeof(loc));
        
        int curStartIdx = -1, max_len = 0;  //curStartIdx为当前子串的开始位置，初始化为-1
        for(int i = 0; i < s.size(); i++)
        {
            if(loc[s[i]] > curStartIdx)  //如果当前字符出现过，那么当前子串的起始位置为这个字符上一次出现的位置+1
            {
                curStartIdx = loc[s[i]];
            }
            if(i - curStartIdx > max_len)  // 使用贪心算法进行子串延伸，关键！！！
            {
                max_len = i - curStartIdx;
            }
            loc[s[i]] = i;  //如果当前字符没出现过，将其位置记录在loc数组中 
        }
        return max_len;
    }
};

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