交叉字符串

给出三个字符串:s1、s2、s3，判断s3是否由s1和s2交叉构成。

比如 s1 = "aabcc" s2 = "dbbca"

    - 当 s3 = "aadbbcbcac"，返回  true.

    - 当 s3 = "aadbbbaccc"， 返回 false.

class Solution {
public:
    /**
     * Determine whether s3 is formed by interleaving of s1 and s2.
     * @param s1, s2, s3: As description.
     * @return: true of false.
     */
    bool isInterleave(string s1, string s2, string s3) {

        int n1=s1.size(),n2=s2.size(),n3=s3.size();

        if(n1>n3||n2>n3) return false;
        int i=0,j=0,k=0;

        if(i==n1) return s3.substr(k)==s2.substr(j);
        if(j==n2) return s3.substr(k)==s1.substr(i);
        if(s3[k]!=s1[i]&&s3[k]!=s2[j]) return false;
        
        return isInterleave(s1.substr(i+1),s2.substr(j),s3.substr(k+1))
        ||isInterleave(s1.substr(i),s2.substr(j+1),s3.substr(k+1));
 

    }
};