18. 4Sum

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http://www.cnblogs.com/yrbbest/p/4434188.html

同3Sum。 Time Complexity - O(n³)， Space Complexity - O(1)

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

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