[LintCode] 587 Two Sum - Unique pairs 解题报告

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



Example
Given nums = [1,1,2,45,46,46], target = 47
return 2

1 + 46 = 47
2 + 45 = 47



思路
老样子，先排序。
一个从左，一个从右，往当中走。
如果加起来等于target，那么左边往右走到不是左边这个数的地方，右边往左走到不是右边这个数字的地方。
然后接着比。



Code

public class Solution {
    /**
     * @param nums an array of integer
     * @param target an integer
     * @return an integer
     */
    public int twoSum6(int[] nums, int target) {
        // Write your code here
        
        if (nums == null || nums.length < 2) {
            return 0;
        }
        
        Arrays.sort(nums);
        
        int count = 0;
        int left = 0;
        int right = nums.length - 1;
        
        while (left < right) {
            if (nums[left] + nums[right] < target) {
                left++;
            }
            else if (nums[left] + nums[right] > target) {
                right--;
            }
            else {
                count++;
                left++;
                right--;
                while (left < right && nums[left] == nums[left - 1]) {
                    left++;
                }
                while (left < right && nums[right] == nums[right + 1]) {
                    right--;
                }
            }
        }
        
        return count;
    }
}