算是最长公共子序列问题吧，mark一下

class Solution {
public:
  bool f[1000][1000];
    bool isInterleave(string s1, string s2, string s3) {
        if(s1.size() + s2.size() != s3.size())
           return false;
         f[0][0] = true;
         for(int i = 1; i <= s1.size(); i++)
           f[i][0] = f[i-1][0] && (s3[i-1] == s1[i-1]);
         for(int j = 1; j <= s2.size(); j++)
           f[0][j] = f[0][j-1] && (s3[j-1] == s2[j-1]);

         for(int i = 1; i <= s1.size(); i++)
           for(int j = 1; j <= s2.size(); j++)
             f[i][j] = (f[i][j-1] && s2[j-1] == s3[i+j-1]) || (f[i-1][j] && s1[i-1] == s3[i+j-1]);

        return f[s1.size()][s2.size()];

    }
};