H-Index和H-Index II

H-Index

原題

Given an array of citations (each citation is a non-negative integer) of a researcher, 
write a function to compute the researcher's h-index.

According to the definition of h-index on Wikipedia: 
"A scientist has index h if h of his/her N papers 
have at least h citations each, and the other N − h 
papers have no more than h citations each."

For example, given citations = [3, 0, 6, 1, 5], 
which means the researcher has 5 papers in total 
and each of them had received 3, 0, 6, 1, 5 citations respectively. 
Since the researcher has 3 papers with at least 3 citations each 
and the remaining two with no more than 3 citations each, his h-index is 3.
Note: If there are several possible values for h, 
the maximum one is taken as the h-index.

思路

直接按wiki上的說來弄.

code

class Solution {
public:
    int hIndex(vector<int>& citations) {
        sort(citations.begin(), citations.end(), greater<int>());
        int i;
        for(i = 0; i < citations.size(); i++) {
            if(citations[i] <= i) return i;
        }
        return i;
    }
};

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H-Index II

原題

Follow up for H-Index: 
What if the citations array is sorted in ascending order? 
Could you optimize your algorithm?

思路

類似二分查找.

code

class Solution {
public:
    int hIndex(vector<int>& citations) {
        int len = citations.size();
        int l = 0, r = len - 1;
        while(l <= r) {
            int m = l + ((r - l) >> 1);
            int m2 = len - m;
            if(m2 <= citations[m]) {
                r = m - 1;
            } else {
                l = m + 1;
            }
        }
        return len - l;
    }
};