class Solution {
public:
    bool isInterleave(string s1, string s2, string s3) {
        int n=s1.size();
        int m=s2.size();
        if(s3.size()!=n+m)return false;
        vector<vector<bool>> f(n+1, vector<bool>(m+1));
        s1=' '+s1;
        s2=' '+s2;
        s3=' '+s3;
        for(int i=0;i<=n;i++)
        {
            for(int j=0;j<=m;j++)
            {
                if(!i&&!j)f[i][j]=true;
                else{
                    if(i&&s1[i]==s3[i+j])f[i][j]=f[i-1][j];
                    if(j&&s2[j]==s3[i+j])f[i][j]=f[i][j]||f[i][j-1];
                }
            }
        }
        return f[n][m];
    }
};