題目

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:

    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)

思路

和3SUM的類似。先排序，然後固定2個位置，然後對剩下的2個一個個匹配。其中要注意的點就是可以直接跳過附近重復的元素，以此來得到獨一無二的結果。

但是這個是n^3的..我本來以為AC不了，結果A了....

請大神指教更好的方法！

代碼

public class Solution {
    public ArrayList> fourSum(int[] num, int target) {
	Arrays.sort(num);
    	ArrayList> result = new ArrayList>();
    	
    	if(num.length==4&&num[0]+num[1]+num[2]+num[3]==target)
    	{
    		ArrayList temp = new ArrayList();
    		temp.add(num[0]);
    		temp.add(num[1]);
    		temp.add(num[2]);
    		temp.add(num[3]);
    		result.add(temp);
    		return result;
    	}
    	
    	for( int a = 0 ; a <=num.length-4; a++)
    	{
    		if(a!=0&&num[a]==num[a-1])
    		{
    			continue;
    		}
    		for(int b = a+1; b<=num.length-3;b++)
    		{
    			if(b-1>a&&num[b]==num[b-1])
    			{
    				continue;
    			}    			
    			int c = b+1;
    			int d = num.length-1;
    			while(c temp = new ArrayList();
    					temp.add(num[a]);
    					temp.add(num[b]);
    					temp.add(num[c]);
    					temp.add(num[d]);
    					result.add(temp);
    					c++;
    					d--;
    					while(c