LeetCode - Longest Common Subsequence - 30Days Challenge

Given two strings text1 and text2, return the length of their longest common subsequence.

A subsequence of a string is a new string generated from the original string with some characters(can be none) deleted without changing the relative order of the remaining characters. (eg, “ace” is a subsequence of “abcde” while “aec” is not). A common subsequence of two strings is a subsequence that is common to both strings.

If there is no common subsequence, return 0.

Example 1:

Input: text1 = “abcde”, text2 = “ace” Output: 3 Explanation: The longest common subsequence is “ace” and its length is 3.

Example 2:

Input: text1 = “abc”, text2 = “abc” Output: 3 Explanation: The longest common subsequence is “abc” and its length is 3.

Example 3:

Input: text1 = “abc”, text2 = “def” Output: 0 Explanation: There is no such common subsequence, so the result is 0.

Constraints:

1 <= text1.length <= 1000
1 <= text2.length <= 1000
The input strings consist of lowercase English characters only.

Solution

Its a standard DP problem to find LCS sequence, Maintain DP matrix for previous states, Use previous states to calculate the current state.

Code


 public class LongestCommonSubSeq {

	public int longestCommonSubsequence(String text1, String text2) {
		int n = text1.length(), m = text2.length();
		int[][] dp = new int[n + 1][m + 1];

		for (int i = 1; i <= n; i++) {
			for (int j = 1; j <= m; j++) {

				if (text1.charAt(i - 1) == text2.charAt(j - 1)) {
					dp[i][j] = dp[i - 1][j - 1] + 1;
				} else {
					dp[i][j] = Math.max(dp[i - 1][j], dp[i][j - 1]);
				}
			}
		}

		return dp[n][m];
	}

	public static void main(String[] args) {
		
		System.out.println(new LongestCommonSubSeq().longestCommonSubsequence("abcde", "ace"));

	}

}