[leetcode]3Sum

给一个数组，球里面所有满足 a+b+c = 0的元素

T_T开始想的是hash，然后查0-a-b这个。

但是发现不好处理重复元素的问题。

T_T，那就这样。。。排序。。。然后二分

先枚举a

for(int i = 0 ; i < cap-2 ; i++)

这样也会有重复的问题，所以下个i不应该是i＋＋而且，刚好大于num[i]的那个位置

同理j也是

然后再用二分查找0-a-b这个数是否存在。

当然为了保证不重复，我们另a <= b <= c ,所以只查找后面的元素

 

class Solution {
public:
    int upper(int start , int end , vector<int> &num , int target){
        int l = start;
        int r = end - 1;
        while(l <= r){
            int mid = (l + r) / 2;
            if(num[mid] <= target) l = mid + 1;
            else r = mid -1; 
        }
        return l;
    }
    bool bsearch(int start , int end , vector<int>&num , int target){
        int l = start;
        int r = end - 1;
        while(l <= r){
            int mid = (l + r) / 2;
            if(num[mid] == target) return true;
            else if(num[mid] > target) r = mid - 1;
            else l = mid + 1;
        }
        return false;
    }
    vector<vector<int> > threeSum(vector<int> &num) {
        // Note: The Solution object is instantiated only once and is reused by each test case.
        vector<vector<int> >ans;
        if (num.size() < 3) return ans;
        sort(num.begin() , num.end());
        int cap = num.size();
        for(int i = 0 ; i < cap - 2 ; i = upper(i , cap-2 , num , num[i])){
            for(int j = i + 1 ; j < cap - 1 ; j =upper(j , cap-1 , num , num[j])){
                if(bsearch(j + 1, cap , num , 0 - num[i] - num[j])){
                    vector<int> tmp;
                    tmp.push_back(num[i]);tmp.push_back(num[j]);tmp.push_back(0 - num[i] - num[j]);
           //         cout << num[i] << " " << num[j] << " " << (0 - num[i] - num[j]) << endl;
                    ans.push_back(tmp);
                }
            }
        }
        return ans;
    }
};

　　

class Solution {
public:
    int up(const vector<int>& num, int left) {
        int l = left;
        int r = num.size() - 1;
        int key = num[left];
        while(l <= r) {
            int mid = l + (r - l) / 2;
            if (num[mid] > key) r = mid - 1;
            else  l = mid + 1;
        }
        return l;
    }
    bool bs(const vector<int>& num, int left, int target) {
        int l = left;
        int r = num.size() - 1;
        while(l <= r) {
            int mid = l + (r - l) / 2;
            if (num[mid] == target) return true;
            if (num[mid] < target) l = mid + 1;
            else r = mid - 1;
        }
        return false;
    }
    vector<vector<int> > threeSum(vector<int> &num) {
        vector<vector<int> > ans;
        if (num.size() < 3) return ans;
        sort(num.begin(), num.end());
        for (int i = 0; i < num.size() - 2; i = up(num, i)) {
            for (int j = i + 1; j < num.size() - 1; j = up(num, j)) {
                bool find = bs(num, j + 1, 0 - num[i] - num[j]);
                if (find) {
                    vector<int> tmp;
                    tmp.push_back(num[i]);tmp.push_back(num[j]);tmp.push_back(0 - num[i] - num[j]);
                    ans.push_back(tmp);
                }
            }
        }
        return ans;
    }
};