Description:
Given an array of integers, find how many pairs in the array such that their sum is less than or equal to a specific target number. Please return the number of pairs.
Example:
Given nums = [2, 7, 11, 15], target = 24.
Return 5.
2 + 7 < 24
2 + 11 < 24
2 + 15 < 24
7 + 11 < 24
7 + 15 < 25
Link:
http://www.lintcode.com/en/problem/two-sum-less-than-or-equal-to-target/
題目解讀：
這道題給定了一個數組與一個target數，要求返回數組中“和”小于或者等于target數的兩個數組合的個數。
解題方法：
首先將數組排序，然后使用頭尾指針解決，當頭尾兩個數相加大于target，尾指針向前移；否則尾指針前面到頭指針所有的數與頭指針的數相加都會小于或等于target，所以count += end - start. 直到頭指針和尾指針相交循環結束。
Time Complexity：
O(nlogn)
完整代碼：
int twoSum5(vector<int> &nums, int target) { int count = 0; if(nums.size() < 2) return count; sort(nums.begin(), nums.end()); int start = 0, end = nums.size() - 1; while(start < end) { int temp = nums[start] + nums[end]; if(temp > target) end--; else { count += end - start; start++; } } return count; }
