3Sum Closest

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class Solution {public:    int threeSumClosest(vector<int> &num, int target)     {        int result;        int gap = INT_MAX;        sort(num.begin(), num.end());                for(int i=0; i<num.size()-2; i++)        {            if(i>0 && num[i] == num[i-1]) continue;                        int left = i+1;            int right = num.size()-1;            while(left < right)            {                auto sum = num[i] + num[left] + num[right];                if(abs(target-sum) < gap)                {                    gap = abs(target-sum);                    result = sum;                }                                    if(sum < target) left++;                else if(sum > target) right--;                else return result; //this branch cannot be neglect, otherwise                 //there would be infinite loop with example [0 1 2] 3            }        }        return result;    }};

排序后左右夹逼，时间O(n^2)。

8.23最新代码：

class Solution {public:    int threeSumClosest(vector<int> &num, int target) {        int gap = INT_MAX;        int real = 0;        int ret = 0;                std::sort(num.begin(), num.end());                for(int i=0; i<num.size()-2; i++)        {            if(i>0 && num[i] == num[i-1])                continue;                        int left = i+1;            int right = num.size()-1;            while(left < right)            {                if(left > i+1 && num[left] == num[left-1])                {                    left++;                    continue;                }                if(right < num.size()-1 && num[right] == num[right+1])                {                    right--;                    continue;                }                                real = num[i] + num[left] + num[right];                if(gap > abs(real-target))                {                    gap = abs(real - target);                    ret = real;                }                if(real < target)                    left++;                else if(real > target)                    right--;                else                {                    return ret;                }            }        }        return ret;    }};

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