C++
class Solution {
public:
bool isInterleave(string s1, string s2, string s3) {
if (s1.size() + s2.size() != s3.size()) return false;
int n1 = s1.size();
int n2 = s2.size();
vector<vector<bool> > dp(n1 + 1, vector<bool> (n2 + 1, false));
dp[0][0] = true;
for (int i = 1; i <= n1; ++i) {
dp[i][0] = dp[i - 1][0] && (s1[i - 1] == s3[i - 1]);
}
for (int i =