LeetCode 16 3Sum Closet

题目

Given an array S of n integers, find three integers in S such that the sum is closest to a given number, target. Return the sum of the three integers. You may assume that each input would have exactly one solution.

Example: given array S = {-1 2 1 -4}, and target = 1. The sum that is closest to the target is 2. (-1 + 2 + 1 = 2).

解法

和题目和3Sum很像，不过不再是求三个数的和是否为0，而是三个数的和与target的最小差，因此需要记录当前最优解并不断更新其值。 该方法时间复杂度为：O(N^2)

class Solution { public: int threeSumClosest(vector<int>& nums, int target) { int closest = 0; int diff = INT_MAX; if (nums.size() < 3) return closest; sort(nums.begin(),nums.end()); for (int i = 0; i < nums.size() - 2; i++) { while(i > 0 && i < nums.size() && nums[i] == nums[i-1]) i++; int start = i + 1, end = nums.size() - 1; while (start < end) { int sum = nums[i] + nums[start] + nums[end]; if (sum == target) { return sum; } else if (sum < target) { if (target - sum < diff) { diff = target - sum; closest = sum; } start++; while(start < end && nums[start] == nums[start-1]) start++; } else { if (sum - target < diff) { diff = sum - target; closest = sum; } end--; while(start < end && nums[end] == nums[end + 1]) end--; } } } return closest; } };