leetcode------4Sum

标题：	4Sum
通过率：	21.4%
难度：	中等

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)

4sum相当于在3sum外加一层循环：直接看代码：

class Solution:
    # @return a list of lists of length 4, [[val1,val2,val3,val4]]
    def fourSum(self, num, target):
        num.sort()
        ans = []
        for j in range(len(num)):
            for i in range(j+1, len(num)):
                l, r = i + 1, len(num) - 1
                while l < r:
                    sum = num[j] + num[i] + num[l] + num[r] 
                    if sum == target:
                        tmp=[num[j],num[i], num[l], num[r]]
                        if tmp not in ans:
                            ans.append(tmp)
                        l, r = l + 1, r - 1
                    elif sum < target:
                        l = l + 1
                    else:
                        r = r - 1   
        return ans