# 0275. H-Index II
###### tags: `Leetcode` `Medium` `Binary Search`
Link: https://leetcode.com/problems/h-index-ii/description/
## 思路
### Binary Search
binary search h index
对于每一个```h```我们要检查到底有多少篇paper的citation大于等于```h``` 这部分也可以用binary search解决
之所以在line4 把end设成```n+1```是因为有可能```h```是答案 所以我们必须要让```start```最大值是```n+1```
## Code

class Solution {
    public int hIndex(int[] citations) {
        int n = citations.length;
        int start = 0, end = n+1;
        while(start<end){
            int mid = start+(end-start)/2;
            if(numOfPaper(mid, citations)>=mid){
                start = mid+1;
            }
            else end = mid;
        }
        return start-1;
    }
    private int numOfPaper(int h, int[] citations){
        int n = citations.length;
        int start = 0, end = n;
        while(start<end){
            int mid = start+(end-start)/2;
            if(citations[mid]<h){
                start = mid+1;
            }
            else end = mid;
        }
        return n-start;
    }
}