Two Sum - Unique pairs Lintcode

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

 

这道题还蛮简单的，就是从两头往中间算就可以了。需要注意的是，把while放在相等的情况里面，可以加快速度。

原始版本：

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

改进版本：

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