[LeetCode] H-Index II 求H指數之二

weixin_34391854發表於2015-09-05
LeetCodeIndex

 

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

Hint:

  1. Expected runtime complexity is in O(log n) and the input is sorted.

 

這題是之前那道H-Index 求H指數的擴充，輸入陣列是有序的，讓我們在O(log n)的時間內完成計算，看到這個時間複雜度，應該有很敏銳的意識應該用二分查詢法，我們最先初始化left和right為0和陣列長度len-1，然後取中間值mid，比較citations[mid]和len-mid做比較，如果前者大，則right移到mid之前，反之right移到mid之後，終止條件是left>right，最後返回len-left即可，參見程式碼如下：

 

class Solution {
public:
    int hIndex(vector<int>& citations) {
        int len = citations.size(), left = 0, right = len - 1;
        while (left <= right) {
            int mid = 0.5 * (left + right);
            if (citations[mid] == len - mid) return len - mid;
            else if (citations[mid] > len - mid) right = mid - 1;
            else left = mid + 1;
        }
        return len - left;
    }
};

 

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