【leetcode刷题笔记】4Sum

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:

• Elements in a quadruplet (a,b,c,d) must be in non-descending order. (ie, a ≤ b ≤ c ≤ d)
• 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)

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题解：参考3Sum，把4Sum问题转换成3Sum，再转换成2Sum，如下图所示：

代码如下：

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