97.交错字符串

学会了DP法，总想试一下递归，毕竟学DP从递归学起的。

DP法显然通过，但是递归肯定会超时，但是过了104/106 显然是对的。 

DP

    public boolean isInterleave(String s1, String s2, String s3) {
        if(s1 == "" && s2 == "" && s3 == "") return true;
        if (s1.length() + s2.length() != s3.length()) {
			return false;
		}
		char[] str1 = s1.toCharArray();
		char[] str2 = s2.toCharArray();
		char[] str3 = s3.toCharArray();
		int n = str1.length;
		int m = str2.length;
        boolean[][] dp= new boolean[n+1][m+1];
        dp[0][0] = true;
        for(int i = 1;i<n+1;i++){
            if(str1[i-1]!=str3[i-1]){
                break;
            }
            dp[i][0] = true;
        }

        for(int i = 1;i<m+1;i++){
            if(str2[i-1] != str3[i-1]){
                break;
            }
            dp[0][i] = true;
        }

        for(int i = 1;i<n+1;i++){
            for(int j = 1;j<m+1;j++){
                dp[i][j] =  (str3[i+j-1] == str1[i-1] && dp[i-1][j])
                            ||
                            (str3[i+j-1] == str2[j-1] && dp[i][j-1]);
            }
        }
        return dp[n][m];
    }

递归

class Solution {
    public boolean isInterleave(String s1, String s2, String s3) {
        char[] char1=s1.toCharArray();
        char[] char2 = s2.toCharArray();
        char[] char3 = s3.toCharArray();
        if(char1.length + char2.length != char3.length){ return false;}
        int i=0,j=0,k=0;
        return f(char1,char2,char3,i,j,k);

    }

    public boolean f(char[] char1,char[] char2,char[] char3,int i,int j,int k){
        if(i ==char1.length && char2.length == j && char3.length==k){
            return true;
        }
        if(i == char1.length && j!= char2.length){
        return char2[j]==char3[k]&f(char1,char2,char3,i,j+1,k+1);
        }
        if(j == char2.length && i != char1.length){
    
        return char1[i]==char3[k]&f(char1,char2,char3,i+1,j,k+1);
        }
        if(char1[i] == char3[k] && char2[j] == char3[k]){
            return true& (f(char1,char2,char3,i+1,j,k+1)|f(char1,char2,char3,i,j+1,k+1));
        }
        if(char1[i] == char3[k]){
            return true& f(char1,char2,char3,i+1,j,k+1);
        }
        if(char2[j] == char3[k]){
            return true& f(char1,char2,char3,i,j+1,k+1);
        }
        if(char1[i] != char3[k] && char2[j] != char3[k]){
            return false;
        }
        return true;
    }
}