bool isInterleave(string s1, string s2, string s3) {

int m = s1.size(), n = s2.size();

if (s3.size() != m +n) return false;

vector<vector<bool> > dp(m+1, vector<bool>(n+1, false));

for (int i = 0; i <=m ;++i) {

for (int j = 0; j <=n; ++j) {

if (i==0 && j ==0) dp[i][j] = true;

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

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

else if (s1[i-1] == s3[i+j-1] && s2[j-1] != s3[i+j-1]) {

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

}

else if (s2[j-1] == s3[i+j-1] && s1[i-1] != s3[i+j-1]){

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

}else if (s1[i-1] == s3[i+j-1] && s2[j-1] == s3[i+j-1]){

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

}

}

}

return dp[m][n];

}