LeetCode875. Koko Eating Bananas——二分答案

一、题目

Koko loves to eat bananas. There are n piles of bananas, the ith pile has piles[i] bananas. The guards have gone and will come back in h hours.

Koko can decide her bananas-per-hour eating speed of k. Each hour, she chooses some pile of bananas and eats k bananas from that pile. If the pile has less than k bananas, she eats all of them instead and will not eat any more bananas during this hour.

Koko likes to eat slowly but still wants to finish eating all the bananas before the guards return.

Return the minimum integer k such that she can eat all the bananas within h hours.

Example 1:

Input: piles = [3,6,7,11], h = 8
Output: 4
Example 2:

Input: piles = [30,11,23,4,20], h = 5
Output: 30
Example 3:

Input: piles = [30,11,23,4,20], h = 6
Output: 23

Constraints:

1 <= piles.length <= 104
piles.length <= h <= 109
1 <= piles[i] <= 109

二、题解

答案在1到数组的最大值之间，并且具有单调性，因此可以在l到r之间对答案进行二分。
注意可能会溢出，因此函数返回值需要使用long long，mid的计算不建议使用(l + r) / 2。

class Solution {
public:
    int minEatingSpeed(vector<int>& piles, int h) {
        int l = 1;
        int r = *max_element(piles.begin(),piles.end());
        int res = 0;
        while(l <= r){
            int mid = l + ((r - l) >> 1);
            if(f(piles,mid) <= h){
                res = mid;
                r = mid - 1;
            }
            else l = mid + 1;
        }
        return res;
    }
    long long f(vector<int>& piles,int speed){
        long long res = 0;
        for(auto p:piles){
            //相当于向上取整
            res += (p + speed - 1) / speed;
        }
        return res;
    }
};