种花， 美团笔试题

一、分治法

时间复杂度：O(NlogN)

通过70%样例，超时

import java.util.*;
public class Main {
    static int res = 0;
    static void solution(int[] a, int i, int j, int x) {
        if(i > j || i < 0 || i >= a.length || j < 0 || j >= a.length) return;
        int idx = i;
        for(int k = i+1; k <= j; k++) 
            if(a[k] < a[idx]) idx = k;
        res += (a[idx]-x);
        
        solution(a, i, idx-1, a[idx]);
        solution(a, idx+1, j, a[idx]);
    }
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int[] a = new int[n];
        for(int i=0; i < n; i++) {
            a[i] = sc.nextInt();
        }
        solution(a, 0, n-1, 0);
        System.out.println(res);
    }
}

二、贪心

时间复杂度： O(N)

import java.util.*;
public class Main {
    static int res = 0;
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int[] a = new int[n];
        for(int i=0; i < n; i++) {
            a[i] = sc.nextInt();
        }
        int res = a[n-1];
        for(int i=1; i < n; i++) {
            if(a[i-1] > a[i])
                res += a[i-1] - a[i];
        }
        System.out.println(res);
    }
}

三、动态规划

时间复杂度：O(N)

import java.util.*;
public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int[] a = new int[n];
        for(int i=0; i < n; i++) {
            a[i] = sc.nextInt();
        }
        int[] f = new int[n];
        f[0] = a[0];
        for(int i=1; i < n; i++) {
            if(a[i] > a[i-1])
                f[i] = f[i-1] + a[i] - a[i-1];
            else
                f[i] = f[i-1];
        }
        System.out.println(f[n-1]);
    }
}
/*
f[i] 表示 完成前i个花园的种花计划最少要天数。
a[i] <= a[i-1],  f[i] = f[i-1]
否则， f[i] = f[i-1] + (a[i] - a[i-1])
*/