97. Interleaving String

class Solution {
    public boolean isInterleave(String s1, String s2, String s3) {
        int n1 = s1.length(), n2 = s2.length();
        if(n1 + n2 != s3.length()) return false;
        boolean[][] dp = new boolean[n1+1][n2+1];
        for(int i = 0; i <= n1; i++){
            for(int j = 0; j <= n2; j++){
                if(i == 0 && j == 0) dp[i][j] = true;
                else if(i == 0){
                    dp[i][j] = dp[i][j - 1] && s2.charAt(j - 1) == s3.charAt(i + j - 1);
                }
                else if(j == 0){
                    dp[i][j] = dp[i - 1][j] && s1.charAt(i - 1) == s3.charAt(i + j - 1);
                }
                else{
                    dp[i][j] = (dp[i - 1][j] && s1.charAt(i - 1) == s3.charAt(i + j - 1)) || (dp[i][j - 1] && s2.charAt(j - 1) == s3.charAt(i + j - 1));
                }
            }
        }
        return dp[n1][n2];
    }
}