Two Sum II

23% Accepted

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.

Example
numbers=[2, 7, 11, 15], target=24

return 1

Challenge

Either of the following solutions are acceptable: O(1) Space, O(nlogn) Time

Tags Expand

• Two Pointers

思路

• easy
• 类似于two sum, 但是画图
• 因为是大于不再是等于了,所以不能再使用hashmap来优化了
• 这个时候采取two sum的另一种做法, 利用排序来做
• 这个题还有一些小trick
• 找到 nums[start] + nums[end] > target 并不是count++, 很容易在这里犯错
• 比如[3,4,5,7,8] target = 9
• start = 0 ,end = 4, 3 + 8 > 9, end--
• 这个时候不是count++,因为4+8 5+8 7+8 都会大于target
• 所以是count = count + end - start

public class Solution {
    /**
     * @param nums: an array of integer
     * @param target: an integer
     * @return: an integer
     */
    public int twoSum2(int[] nums, int target) {
        // Write your code here
        if (nums == null || nums.length < 2) {
            return 0;
        }

        Arrays.sort(nums);
        int count = 0;

        int start = 0;
        int end = nums.length - 1;

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

        return count;
    }
}