给出三个字符串:s1、s2、s3，推断s3是否由s1和s2交叉构成。

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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) {

        // write your code here

        if(s3.length()!=s1.length()+s2.length())

            return false;

        if(s1.length()==0)

            return s2==s3;

        if(s2.length()==0)

            return s1==s3;

        vector<vector<bool> > dp(s1.length()+1,vector<bool>(s2.length()+1,false));

        dp[0][0] = true;

        for(int i=1;i<=s1.length();i++)

            dp[i][0] = dp[i-1][0]&&(s3[i-1]==s1[i-1]);

        for(int i=1;i<=s2.length();i++)

            dp[0][i] = dp[0][i-1]&&(s3[i-1]==s2[i-1]);

        for(int i=1;i<=s1.length();i++)

        {

            for(int j=1;j<=s2.length();j++)

            {

                int t = i+j;

                if(s1[i-1]==s3[t-1])

                    dp[i][j] = dp[i][j]||dp[i-1][j];

                if(s2[j-1]==s3[t-1])

                    dp[i][j] = dp[i][j]||dp[i][j-1];

            }

        }

        return dp[s1.length()][s2.length()];

    }

};