Leetcode -- Interleaving String

class Solution {  
 public:  
   bool isInterleave(string s1, string s2, string s3) {  
     int n1 = s1.size();  
     int n2 = s2.size();  
     int n3 = s3.size();  
     if(n1 + n2 != n3) {  
       return false;  
     }  
     vector<vector<bool> > dp(n1 + 1, vector<bool>(n2 + 1, 0));  
     dp[0][0] = 1;  
     for(int i = 1; i <=n1; ++i) {  
       if(s1[i - 1] == s3[i - 1] && dp[i - 1][0]) {  
         dp[i][0] = 1;  
       }  
     }  
     for(int j = 1; j <= n2; ++j) {  
       if(s2[j - 1] == s3[j - 1] && dp[0][j - 1]) {  
         dp[0][j] = 1;  
       }  
     }  
     for(int i = 1; i <= n1; ++i) {  
       for(int j = 1; j <= n2; ++j) {  
         dp[i][j] = dp[i - 1][j] && s1[i - 1] == s3[i + j - 1] || dp[i][j - 1] && s2[j - 1] == s3[i + j - 1];  
       }  
     }  
     return dp[n1][n2];  
   }  
 };