leetcode 139. Word Break ----- java

Given a string s and a dictionary of words dict, determine if s can be segmented into a space-separated sequence of one or more dictionary words.

For example, given
s = "leetcode",
dict = ["leet", "code"].

Return true because "leetcode" can be segmented as "leet code".

查看字符串是否由字典中的单词组成。

1、暴力递归，果断超时。

public class Solution {
    public boolean wordBreak(String s, Set<String> wordDict) {

        return helper(s,0,wordDict);

    }
    public boolean helper(String s,int start,Set<String> wordDict ){

        if( start == s.length() )
            return true;

        for( int i = s.length();i > start ;i--){

            if( wordDict.contains( s.substring(start,i) ) ){
                if( helper(s,i,wordDict) )
                    return true;
            }

        }
        return false;
    }
}

2、dp，基本达到最快。

        int len = s.length(),maxLen = 0;
        boolean[] dp = new boolean[len];
        for( String str : wordDict ){
            maxLen = Math.max(maxLen,str.length());
        }

        for( int i = 0 ;i<len;i++){

            for( int j = i;j>=0 && i-j<maxLen;j-- ){
                if( ( j == 0 || dp[j-1] == true ) && wordDict.contains(s.substring(j,i+1)) ){
                    dp[i] = true;
                    break;
                }
            }
        }

        return dp[len-1];