Lintcode 443 · Two Sum - Greater than target (binary search经典题)

443 · Two Sum - Greater than target
Medium

Description
Given an array of integers, find how many pairs in the array such that their sum is bigger than a specific target number. Please return the number of pairs.

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Example
Example 1:

Input: [2, 7, 11, 15], target = 24
Output: 1
Explanation: 11 + 15 is the only pair.
Example 2:

Input: [1, 1, 1, 1], target = 1
Output: 6
Challenge
Do it in O(1) extra space and O(nlogn) time.

解法1：

class Solution {
public:
    /**
     * @param nums: an array of integer
     * @param target: An integer
     * @return: an integer
     */
    int twoSum2(vector<int> &nums, int target) {
        int n = nums.size();
        sort(nums.begin(), nums.end());
        int start = 0, end = n - 1;
        int count = 0;
        while (start < end) {
            int sum = nums[start] + nums[end];
            if (sum > target) {
                count += end - start;
                end--;
            } else if (sum <= target) {
                start++;
            }
        }
        return count;
    }
};