JAVA代码—算法基础：查找最长无重复的子字符串

查找最长无重复的子字符串

题目描述：给定一个字符串，查找最长无重复的子字符串的长度

例子：

• 给定字符串 “abcabcbb”，最长无重复子字符串为“abc”，长度为3
• 给定字符串“bbbbb”，最长无重复子字符串为“b”，长度为1
• 给定字符串“pwwkew”，最长无重复子字符串为“wke”，长度为3

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.

算法设计一：暴力求解 （Brute Force）

采用暴力求解，但是时间超时（Time Limit Exceeded），不是最优算法。

public static int lengthOfLongestSubstring(String s) {
        int n = s.length();
        int ans = 0;
        for (int i = 0; i < n; i++)
            for (int j = i + 1; j <= n; j++)
                if (allUnique(s, i, j)) ans = Math.max(ans, j - i);
        return ans;
    }

    public static boolean allUnique(String s, int start, int end) {
        Set set = new HashSet<>();
        for (int i = start; i < end; i++) {
            Character ch = s.charAt(i);
            if (set.contains(ch)) return false;
            set.add(ch);
        }
        return true;
    }

完整的代码如下：

package com.bean.algorithmbasic;

import java.util.HashSet;
import java.util.Set;

public class LongestSubStringNoRepeat {

    public static void main(String[] args) {
        // TODO Auto-generated method stub

        /*
         * 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.
         * */

        String strDemo="abcabcbb";
        int result=lengthOfLongestSubstring(strDemo);
        System.out.println(result);

    }

    public static int lengthOfLongestSubstring(String s) {
        int n = s.length();
        int ans = 0;
        for (int i = 0; i < n; i++)
            for (int j = i + 1; j <= n; j++)
                if (allUnique(s, i, j)) ans = Math.max(ans, j - i);
        return ans;
    }

    public static boolean allUnique(String s, int start, int end) {
        Set set = new HashSet<>();
        for (int i = start; i < end; i++) {
            Character ch = s.charAt(i);
            if (set.contains(ch)) return false;
            set.add(ch);
        }
        return true;
    }

}

算法设计二：设定滑动窗口（Sliding Window）

public int lengthOfLongestSubstring(String s) {
        int n = s.length();
        Set set = new HashSet<>();
        int ans = 0, i = 0, j = 0;
        while (i < n && j < n) {
            // try to extend the range [i, j]
            if (!set.contains(s.charAt(j))){
                set.add(s.charAt(j++));
                ans = Math.max(ans, j - i);
            }
            else {
                set.remove(s.charAt(i++));
            }
        }
        return ans;
    }

算法设计三：对滑动窗口的优化算法

public int lengthOfLongestSubstring(String s) {
        int n = s.length(), ans = 0;
        Map<Character, Integer> map = new HashMap<>(); // current index of character
        // try to extend the range [i, j]
        for (int j = 0, i = 0; j < n; j++) {
            if (map.containsKey(s.charAt(j))) {
                i = Math.max(map.get(s.charAt(j)), i);
            }
            ans = Math.max(ans, j - i + 1);
            map.put(s.charAt(j), j + 1);
        }
        return ans;
    }

（完）