wenbao与二分加贪心

https://vjudge.net/contest/144453#problem/A

　　长城守卫

#include <iostream>
#include <stdio.h>
using namespace std;
const int maxn = 1e5+10;
int n, a[maxn], le[maxn], ri[maxn];
int Max(int x, int y) {return x > y ? x : y; }
int Min(int x, int y) {return x < y ? x : y; }
bool ok(int x){
    le[1] = a[1], ri[1] = 0;
    int y = x - a[1];
    for(int i = 2; i <= n; i++){
        if(i&1){
            ri[i] = Min(a[i], y - ri[i-1]);
            le[i] = a[i] - ri[i];
        }else{
            le[i] = Min(a[i], a[1] - le[i-1]);
            ri[i] = a[i] - le[i];
        }
    }
    if(le[n] == 0) return true;
    else return false;
}
int main(){
    while(scanf("%d", &n)&&n){
        for(int i = 1; i <= n; i++) scanf("%d", a+i);
        int L = -1, R = -1;
        a[n+1] = a[1];
        for(int i = 1; i <= n; i++){
            L = Max(L, a[i]+a[i+1]), R = Max(R, a[i]*3);
        }
        if(n == 1) {
            printf("%d
", a[1]);
            continue;
        }
        if(n&1){
            while(L < R){
                int mid = (L+R) >> 1;
                if(ok(mid)){
                    R = mid;
                }else{
                    L = mid + 1;
                }
            }
        }
        printf("%d
", L);
    }
    return 0;
}

只有不断学习才能进步！