Leetcode||18. 4Sum

18. 4Sum

• Total Accepted: 94146
• Total Submissions: 371949
• Difficulty: Medium
• Contributors: Admin

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]
]

这题和15题，16题类似，排序，前后夹逼可以获取全部情况，就是一个动态规划

代码如下：

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