Lintcode:Longest Common Subsequence 解题报告

Longest Common Subsequence

原题链接：http://lintcode.com/zh-cn/problem/longest-common-subsequence/

Given two strings, find the longest comment subsequence (LCS).

Your code should return the length of LCS.

样例
For "ABCD" and "EDCA", the LCS is "A" (or D or C), return 1

For "ABCD" and "EACB", the LCS is "AC", return 2

说明
What's the definition of Longest Common Subsequence?

* The longest common subsequence (LCS) problem is to find the longest subsequence common to all sequences in a set of sequences (often just two). (Note that a subsequence is different from a substring, for the terms of the former need not be consecutive terms of the original sequence.) It is a classic computer science problem, the basis of file comparison programs such as diff, and has applications in bioinformatics.

* https://en.wikipedia.org/wiki/Longest_common_subsequence_problem

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SOLUTION 1:

DP. 

1. D[i][j] 定义为s1, s2的前i,j个字符串的最长common subsequence. 

2. D[i][j] 当char i == char j， D[i - 1][j - 1] + 1

    当char i != char j, D[i ][j - 1], D[i - 1][j] 里取一个大的（因为最后一个不相同，所以有可能s1的最后一个字符会出现在s2的前部分里，反之亦然。

public class Solution {
    /**
     * @param A, B: Two strings.
     * @return: The length of longest common subsequence of A and B.
     */
    public int longestCommonSubsequence(String A, String B) {
        // write your code here
        if (A == null || B == null) {
            return 0;
        }
        
        int lenA = A.length();
        int lenB = B.length();
        int[][] D = new int[lenA + 1][lenB + 1];
        
        for (int i = 0; i <= lenA; i++) {
            for (int j = 0; j <= lenB; j++) {
                if (i == 0 || j == 0) {
                    D[i][j] = 0;
                } else {
                    if (A.charAt(i - 1) == B.charAt(j - 1)) {
                        D[i][j] = D[i - 1][j - 1] + 1;
                    } else {
                        D[i][j] = Math.max(D[i - 1][j], D[i][j - 1]);
                    }
                }
            }
        }
        
        return D[lenA][lenB];
    }
}

 

 https://github.com/yuzhangcmu/LeetCode_algorithm/blob/master/lintcode/dp/longestCommonSubsequence.java