LeetCode 刷题之路 Longest Substring Without Repeating Characters

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.

一开始的想法是通过利用HashMap来判断当前字符是否存在于子字符串中。同时为了让算法更加快速，每次找到重复字符时，直接以这个字符作为新的起始点来进行查找判断。

class Solution {
    public int lengthOfLongestSubstring(String s) {
        int result = 0;
        Map<Character,Integer> map = new HashMap<>();
        int i = 0;
        for (int j = 0; j < s.length(); j++){
            if(map.containsKey(s.charAt(j))){
                i = Math.max(map.get(s.charAt(j)),i);
            }
            result = Math.max(result, j-i+1);
            map.put(s.charAt(j), j + 1);
        }
        return result;
    }
}

后面在官网找到了更优的解法，利用数组直接代替HashMap。

public class Solution {
    public int lengthOfLongestSubstring(String s) {
        int n = s.length(), ans = 0;
        int[] index = new int[128]; // current index of character
        // try to extend the range [i, j]
        for (int j = 0, i = 0; j < n; j++) {
            i = Math.max(index[s.charAt(j)], i);
            ans = Math.max(ans, j - i + 1);
            index[s.charAt(j)] = j + 1;
        }
        return ans;
    }
}