HackerRank# The Longest Common Subsequence

原题地址

LCD，经典动归，O(n^2)复杂度

因为要输出子序列，所以啰嗦一些

#include <cmath>
#include <cstdio>
#include <vector>
#include <iostream>
#include <algorithm>
#include <cstring>
using namespace std;

#define MAX_LEN 128

int n, m;
int a[MAX_LEN], b[MAX_LEN];
int lcd[MAX_LEN][MAX_LEN];
int start_index_a[MAX_LEN][MAX_LEN];
int start_index_b[MAX_LEN][MAX_LEN];

int main() {
    /* Enter your code here. Read input from STDIN. Print output to STDOUT */   
    cin >> n >> m;
    for (int i = 0; i < n; i++)
        cin >> a[i];
    for (int i = 0; i < m; i++)
        cin >> b[i];
    memset(lcd, 0, sizeof(lcd));
    memset(start_index_a, 0, sizeof(start_index_a));
    for (int j = 0; j < m; j++)
        start_index_a[n][j] = start_index_b[n][j] = -1;
    for (int i = 0; i < n; i++)
        start_index_a[i][m] = start_index_b[i][m] = -1;
    for (int i = n - 1; i >= 0; i--) {
        for (int j = m - 1; j >= 0; j--) {
            if (a[i] == b[j]) {
                lcd[i][j] = lcd[i + 1][j + 1] + 1;
                start_index_a[i][j] = i;
                start_index_b[i][j] = j;
            } else if (lcd[i + 1][j] > lcd[i][j + 1]) {
                lcd[i][j] = lcd[i + 1][j];
                start_index_a[i][j] = start_index_a[i + 1][j];
                start_index_b[i][j] = start_index_b[i + 1][j];
            } else {
                lcd[i][j] = lcd[i][j + 1];
                start_index_a[i][j] = start_index_a[i][j + 1];
                start_index_b[i][j] = start_index_b[i][j + 1];
            }
        }
    }
    
    int i = start_index_a[0][0];
    int j = start_index_b[0][0];
    while (i >= 0 && j >= 0) {
        cout << a[i] << " ";
        int ii = start_index_a[i + 1][j + 1];
        int jj = start_index_b[i + 1][j + 1];
        i = ii;
        j = jj;
    }
    cout << endl;
    return 0;
}