AcWing 1012. 友好城市(最长上升子序列)

时间复杂度：

O(n^2);换成单调栈O(nlogn)

算法思想：

对一边排序，另一边求最长上升子序列

//一岸从小到大排序，另一岸求最长上升子序列
import java.util.*;

class Main{
    
    static int n = 0, N = 5010;
    static int[][] nums = new int[N][2];
    static int[] max = new int[N];
    public static void main(String[] args)throws Exception{
        Scanner sc = new Scanner(System.in);
        n = sc.nextInt();
        for(int i = 1; i <= n; ++i){
            nums[i][0] = sc.nextInt();
            nums[i][1] = sc.nextInt();
        }
        Arrays.sort(nums, 1, n + 1, (a, b) -> a[0] - b[0]);
        int res = 0;
        for(int i = 1; i <= n; ++i){
            max[i] = 1;
            for(int j = 1; j <= i; ++j){
                if(nums[i][1] > nums[j][1])
                    max[i] = Math.max(max[i], max[j] + 1);
                res = Math.max(res, max[i]);
            }
        }
        System.out.print(res);
        
    }
}