每日算法之十六：4sum

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)

  直接上代碼：

  最主要的還是進行去重。

  class Solution {
    public:
      vector > fourSum(vector &num, int target) {
           vector > ret;
          if(num.size()==0) return ret;
          sort(num.begin(),num.end());
          for(int i = 0;i0&&num[i] == num[i-1])
                  continue;
              for(int j = i+1;ji+1&&num[j] == num[j-1])
                      continue;
                  int k = j+1,t = num.size()-1;
                  while(kj+1&&num[k] == num[k-1])//這裡是if，而不是while.如果是while可能會越界，在每一次
                          {k++;continue;}     //指針變化之後都要進行判斷。避免越界的發生。
                 //這裡是contimue，而不是break.如果是while,也不能是break，因為可能導致t指針的重復。
   if(ttarget) t--; else { vector v; v.push_back(num[i]); v.push_back(num[j]); v.push_back(num[k]); v.push_back(num[t]);
   ret.push_back(v); k++;//哪一個變化都可以 } } } } return ret; }};