18.4-sum 四数之和

参考三数之和，相比三数之和，只是外面再套了一层循环，即两层循环内部使用双指针法。
同时要注意int 的取值范围，转换成long long，防止溢出。

#include <algorithm>
#include <vector>
using std::vector;
class Solution {
  public:
    vector<vector<int>> fourSum(vector<int> &nums, int target) {
        // vector<long long> nums;
        sort(nums.begin(), nums.end());
        vector<vector<int>> res;
        long long mtarget = target;
        if (nums.size() < 4)
            return res;
        for (int i = 0; i < nums.size() - 3; i++) {
            if (i > 0 && nums[i - 1] == nums[i])
                continue;
            for (int j = i + 1; j < nums.size() - 2; j++) {
                if (j > i + 1 && nums[j - 1] == nums[j])
                    continue;
                int left = j + 1;
                int right = nums.size() - 1;
                while (left < right) {
                    if ((long long)nums[i] + (long long)nums[j] + (long long)nums[left] + (long long)nums[right] < mtarget) {
                        left++;
                        while (nums[left - 1] == nums[left] && left < right - 1)
                            left++;
                    } else if ((long long)nums[i] + (long long)nums[j] + (long long)nums[left] + (long long)nums[right] > mtarget) {
                        right--;
                        while (nums[right + 1] == nums[right] && left < right - 1)
                            right--;
                    } else {
                        res.push_back({nums[i], nums[j], nums[left], nums[right]});
                        left++;
                        while (nums[left - 1] == nums[left] && left < right - 1)
                            left++;
                        right--;
                        while (nums[left - 1] == nums[left] && left < right - 1)
                            right--;
                    }
                }
            }
        }
        return res;
    }
};

标签：四数,nums,int,sum,long,right,&&,18.4,left
来源： https://www.cnblogs.com/zwyyy456/p/16561336.html