4Sum

题目描述：

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)

思路和前面的2sum,3sum差不多,先排序，关键就是最后的要把相同的全部都去掉。

这里的indexOf源码：

<pre name="code" class="java">public int indexOf(Object o) {
    if (o == null) {
        for (int i = 0; i < size; i++)
            if (elementData[i]==null)
                return i;
    } else {
        for (int i = 0; i < size; i++)
            if (o.equals(elementData[i]))
                return i;
    }
    return -1;
}

equals源码：

public boolean equals(Object o) {
    if (o == this)
        return true;
    if (!(o instanceof List))
        return false;

    ListIterator<E> e1 = listIterator();
    ListIterator<?> e2 = ((List<?>) o).listIterator();
    while (e1.hasNext() && e2.hasNext()) {
        E o1 = e1.next();
        Object o2 = e2.next();
        if (!(o1==null ? o2==null : o1.equals(o2)))
            return false;
    }
    return !(e1.hasNext() || e2.hasNext());
}

所以当list中查找是按照equals方法来查找的，而equals又支持List，只要每个元素相等就返回true。所以可以这样去除List<List<Integer>>中重复的list

代码如下：

public class Solution {
    public List> fourSum(int[] nums, int target) {
		List> result=new ArrayList>();
		if(nums==null)
			return result;
		if(nums.length<4)
			return result;
		Arrays.sort(nums);
		int n=nums.length;
		for(int i=0;i<=n-4;i++) {
			for(int j=i+1;j<=n-3;j++){
				int sum=target-nums[i]-nums[j];
				int start=j+1,end=n-1;
				while(start list=new ArrayList();
						list.add(nums[i]);
						list.add(nums[j]);
						list.add(nums[start]);
						list.add(nums[end]);
						result.add(list);
					}
					if(nums[start]+nums[end]<=sum)
						start++;
					else
						end--;
				}
			}
		}
		List> result2 = new ArrayList>();
		for (int s = 0; s < result.size(); s++) {
			List list = result.get(s);
			if (result2.indexOf(list) == -1) {
				result2.add(list);
			}
		}
		return result2;
    }
	
	public boolean equals(List list1,List list2){
		for(int i=0;i