// Complete the commonChild function below.
    static int commonChild(String s1, String s2) {
        int N = s1.length();
        int[][] cache = new int[N+1][N+1];
        for (int i = 0; i < N+1; i++)
            Arrays.fill(cache[i], 0);

        for (int i1 = N-1; i1 >= 0; i1--) {
            for (int i2 = N-1; i2 >= 0; i2--) {

                if (s1.charAt(i1) == s2.charAt(i2)) {
                    cache[i1][i2] = 1 + cache[i1+1][i2+1];
                }
                else cache[i1][i2] = Math.max(cache[i1][i2+1], cache[i1+1][i2]);

            }
        }
        return cache[0][0];