leetcode:3sum closet

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题目：给一个数组和给定的目标值，要求在数组里找出三个元素，这三个元素的和最接近目标值，当然等于是最好的。

用3sum的方法，把判定条件作些修改。

int twoSum(vector<int> &num, int start, int target){if(num.size() < 3 || start >= num.size())return -target;int head = start;int tail = num.size() - 1;int result = -target;int erro = 0x7fffffff;while (head < tail){int sum = num[head] + num[tail];if(sum == target){result = target;return target;    }else if(sum < target){do {++head;} while (head < tail && num[head] == num[head - 1]);}else{do {--tail;} while (tail > head && num[tail] == num[tail + 1]);}int curerro = abs(sum - target);if(curerro < erro){result = sum;erro = curerro;}}return result;}int threeSumClosest(vector<int> &num, int target) {const int len = num.size();if(len < 3)return 0;sort(num.begin(), num.end());int closet = -target;int erro = 0x7fffffff;for (int i = 0; i < len - 2; ++i){int thisloop = twoSum(num, i + 1, target - num[i]);if(thisloop == target - num[i])return target;int cur_erro = abs(thisloop + num[i] - target);    if(cur_erro < erro)  {  closet = thisloop + num[i];  erro = cur_erro;}while(i + 1 < len && num[i + 1] == num[i]) ++i;}return closet;}

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