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

思路

1. 把数组排序。
2. 先把首尾相加，如果小于等于target，就说明每个数和第一个数相加都是小于target的，这个时候count += end - start。且此时start++ 继续进行计算。
3. 如果大于target，就说明最后一个数太大了，end--。

public class Solution {
    /*
     * @param nums: an array of integer
     * @param target: an integer
     * @return: an integer
     */
    public int twoSum5(int[] nums, int target) {
        // write your code here
        if (nums == null || nums.length == 0) {
            return 0;
        }
        
        int start = 0;
        int end = nums.length - 1;
        int count = 0;

        Arrays.sort(nums);
        while (start < end) {
            if (nums[start] + nums[end] > target) {
                end--;
            } else {
                count = count + (end - start);
                start++;
            }
        }
        return count;
    }
}