Common Child in Algorithm | HackerRank Programming Solutions | HackerRank Problem Solving Solutions in Java [💯Correct]

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Introduction To Algorithm

The word Algorithm means “a process or set of rules to be followed in calculations or other problem-solving operations”. Therefore Algorithm refers to a set of rules/instructions that step-by-step define how a work is to be executed upon in order to get the expected results. 

Advantages of Algorithms:

  • It is easy to understand.
  • Algorithm is a step-wise representation of a solution to a given problem.
  • In Algorithm the problem is broken down into smaller pieces or steps hence, it is easier for the programmer to convert it into an actual program.

Link for the ProblemCommon Child – Hacker Rank Solution

Common Child – Hacker Rank Solution


A string is said to be a child of a another string if it can be formed by deleting 0 or more characters from the other string. Letters cannot be rearranged. Given two strings of equal length, what’s the longest string that can be constructed such that it is a child of both?


These strings have two children with maximum length 3, ABC and ABD. They can be formed by eliminating either the D or C from both strings. Return .

Function Description

Complete the commonChild function in the editor below.

commonChild has the following parameter(s):

  • string s1: a string
  • string s2: another string


  • int: the length of the longest string which is a common child of the input strings

Input Format

There are two lines, each with a string,  and .


  •  where  means “the length of “
  • All characters are upper case in the range ascii[A-Z].

Sample Input


Sample Output



The longest string that can be formed by deleting zero or more characters from  and  is , whose length is 2.

Sample Input 1


Sample Output 1


Explanation 1

 and  have no characters in common and hence the output is 0.

Sample Input 2


Sample Output 2


Explanation 2

The longest string that can be formed between  and  while maintaining the order is .

Sample Input 3


Sample Output 3


Explanation 3
 is the longest child of the given strings.

Common Child – Hacker Rank Solution
import java.util.Scanner;

public class CommonChild {

	static int commonChild(String s1, String s2) {
		return LCSM4(s1.toCharArray(), s2.toCharArray(), s1.length(), s2.length());


	public static int LCSM4(char[] X, char[] Y, int m, int n) {
		int memo[] = new int[n + 1];

		for (int i = 1; i <= m; i++) {
			int prev = 0;
			for (int j = 1; j <= n; j++) {
				int temp = memo[j];
				if (X[i - 1] == Y[j - 1]) {
					memo[j] = prev + 1;
				} else {
					memo[j] = Math.max(memo[j], memo[j - 1]);
				prev = temp;

		return memo[n];

	public static void main(String[] args) {
		Scanner sc = new Scanner(;
		String s1 =;
		String s2 =;
		int result = commonChild(s1, s2);


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