609.Two Sum - Less than or equal to target

1.Description(Medium)

Given an array of integers, find how many pairs in the array such that their sum isless than or equal toa specific target number. Please return the number of pairs.

Example

Given nums =[2, 7, 11, 15], target =24. Return5. 2 + 7 < 24 2 + 11 < 24 2 + 15 < 24 7 + 11 < 24 7 + 15 < 25

2.Code

跟two sum很像，一旦发现sum<target就直接把tail-head全部加进去，再head++;

 public int twoSum5(int[] nums, int target) {
        if(nums==null || nums.length<2){
            return 0;
        }
        int result=0;
        int head=0;
        int tail=nums.length-1;
        Arrays.sort(nums);
        while(head<tail){
            int sum=nums[head]+nums[tail];
            if(sum>target){
                tail--;
            }else{
                result=result+(tail-head);
                head++;
            }
        }
        return result;

    }