public static int commonChild(String s1, String s2) {
    int n = s1.length();
    int m = s2.length();
    
    int[][] dp = new int[n+1][m+1];
    
    for (int i = 0; i < n; i++) {
        for (int j = 0; j < m; j++) {
            if (s1.charAt(i) == s2.charAt(j)) {
                dp[i+1][j+1] = dp[i][j] + 1;
            } else {
                dp[i+1][j+1] = Math.max(dp[i+1][j], dp[i][j+1]);
            }
        }
    }
    return dp[n][m];
}