四数之和

题目

代码

public class Solution {
    /**
     * @param numbers: Give an array
     * @param target: An integer
     * @return: Find all unique quadruplets in the array which gives the sum of zero
     */
    public List<List<Integer>> fourSum(int[] numbers, int target) {
        // write your code here
        List<List<Integer>> lists = new LinkedList<>();
        if(numbers==null){
            return lists;
        }
        Arrays.sort(numbers);
        for(int i=0;i<numbers.length;i++){
            if(i>0&&numbers[i]==numbers[i-1]){
                continue;
            }
            for(int j=i+1;j<numbers.length;j++){
                if(j>i+1&&numbers[j]==numbers[j-1]){
                    continue;
                }
                for(int m=j+1,n=numbers.length-1;m<n;){
                    if (m>j+1 && numbers[m-1]==numbers[m]){
                        ++m;                        // duplication
                        continue;
                    }
                    if (n<numbers.length-1&& numbers[n]==numbers[n+1]){
                        --n;                        // duplication
                        continue;
                    }
                    int sum=numbers[i]+numbers[j]+numbers[m]+numbers[n];
                    if(sum>target){
                        n--;
                    }else if(sum<target){
                        m++;
                    }else {
                        List<Integer> temp=new LinkedList<>();
                        temp.add(numbers[i]);temp.add(numbers[j]);temp.add(numbers[m]);temp.add(numbers[n]);
                        m++;
                        lists.add(temp);
                    }
                    
                }
            }
        }
        return lists;
    }
}