【leetcode】3Sum Closest

class Solution {
public:
    int threeSumClosest(vector<int> &num, int target) {
        // Note: The Solution object is instantiated only once and is reused by each test case.
        int ans;
        bool first=true;//第一個3元組
        
        sort(num.begin(),num.end());//sort，O(n*logn)
        
        for(int i=0;i<=num.size()-3;i++)//O(n)
        {//允許3元組中有重複的數字，這點和3Sum不一樣
            int sta=i+1,end=num.size()-1;
            while(sta<end)//O(n)
            {
                int sum=num[i]+num[sta]+num[end];
                
                if(first)//更新ans
                {
                    ans=sum;
                    first=false;
                }
                else
                {
                    ans=(abs(target-ans)<abs(target-sum))?ans:sum;
                }
                
                if(ans==target)//處理返回情況時，是ans與target比較
                    return ans;
                else if(sum<target)//注意此處縮小範圍時，是sum與target比較
                    sta++;
                else end--;    
            }
            
        }    
        return ans;
    }
};