public class Solution {
    /**
     * Determine whether s3 is formed by interleaving of s1 and s2.
     * @param s1, s2, s3: As description.
     * @return: true or false.
     */
    public boolean isInterleave(String s1, String s2, String s3) {
        // write your code here
        if (s1.length() + s2.length() < s3.length()) {
            return false;
        }
        int p = s1.length();
        int q = s2.length();
        boolean[][] dp = new boolean[p + 1][q + 1];
        dp[0][0] = true;
        for (int i = 1; i <= p; i++) {
            if (dp[i - 1][0] && s1.charAt(i - 1) == s3.charAt(i - 1)) {
                dp[i][0] = true;
            }
        }
        for (int i = 1; i <= q; i++) {
            if (dp[0][i - 1] && s2.charAt(i - 1) == s3.charAt(i - 1)) {
                dp[0][i] = true;
            }
        }
        for (int i = 1; i <= p; i++) {
            for (int j = 1; j <= q; j++) {
                if (dp[i - 1][j] && s1.charAt(i - 1) == s3.charAt(i + j - 1)) {
                    dp[i][j] = true;
                }
                if (dp[i][j - 1] && s2.charAt(j - 1) == s3.charAt(i + j - 1)) {
                    dp[i][j] = true;
                }
            }
        }
        return dp[p][q];
    }
}