LeetCode 18. 4Sum

Question

Given an array S of n integers, are there elements a, b, c, and d in S such that a + b + c + d = target? Find all unique quadruplets in the array which gives the sum of target.

Note: The solution set must not contain duplicate quadruplets.

For example, given array S = [1, 0, -1, 0, -2, 2], and target = 0.

A solution set is: [ [-1, 0, 0, 1], [-2, -1, 1, 2], [-2, 0, 0, 2] ]

Explanation

分解成3sum和2sum来做，注意排序以后要跳过重复的值。

Code

public class Solution {
    public List<List<Integer>> fourSum(int[] nums, int target) {
        List<List<Integer>> result = new ArrayList<>();
        if (nums == null || nums.length < 4) return result;
        Arrays.sort(nums);
        for(int i=0; i<nums.length-3; i++) {
            if (i != 0 && nums[i] == nums[i-1]){
                continue;
            }
            for (int j = i+1; j < nums.length-2; j++) {
                if (j>i+1 && nums[j] == nums[j-1]) continue;
                int low = j+1;
                int high = nums.length-1;
                while(low < high) {
                    int sum = nums[i] + nums[j] + nums[low] + nums[high];
                    if(sum == target) {
                        result.add(Arrays.asList(nums[i], nums[j], nums[low], nums[high]));
                    while(low < high && nums[low] == nums[low+1]) low++;
                    while(low < high && nums[high] == nums[high - 1]) high--;
                    low ++;
                    high --;
                    } else if(sum < target) low++;
                    else high --;
                }
            }
        }
        return result;
    }
}