Create 16. 3Sum Closest.md

16. 3Sum Closest.md

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+# [16. 3Sum Closest](https://leetcode.com/problems/3sum-closest/)
+# 思路
+类似[15. 3Sum](https://leetcode.com/problems/3sum/),先对数组进行排序，外层循环就是遍历一遍数组，内层循环就是用两个指针low和high，与3sum不同的就是
+每次循环要判断记录当前sum和target的差值。
+时间复杂度O(n^2)
+
+# C++
+``` C++
+class Solution {
+public:
+    int threeSumClosest(vector<int>& nums, int target) {
+        int len = nums.size();
+        long long res=nums[0] + nums[1] + nums[2], min_gap, sum, curr_gap; // 防止溢出所以用long long型
+        min_gap = abs(res - target);
+        sort(nums.begin(), nums.end());
+        for(int i = 0; i < len - 2; i++){
+            int low = i + 1, high = len - 1;
+            while(low < high){
+                sum = nums[i] + nums[low] + nums[high];
+                curr_gap = abs(sum - target);
+                if(curr_gap < min_gap){
+                    res = sum;
+                    min_gap = curr_gap;
+                } 
+                if(target < sum) high--;
+                else if(target > sum) low++;
+                else return target;
+            } // low == high
+        }
+        return res;   
+    }
+};
+```