某数字在排序数组中出现的次数

二分查找

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剑指offer 上给出的解法：两个递归（循环也可以），找左边界和右边界。
但貌似时间消耗没有 先二分查找到 k ,再左右顺序遍历 少。没想清楚原因。

class Solution {
public:
    int GetNumberOfK(vector<int> data ,int k) {
        if (data.size()==0 )return 0;
        if (data.size()==1)
        {
        	if (k!=data[0])return 0;
        	else return 1;
		}
		
        int ans = 0;
        
        int left = GetFirstK(data,k,0,data.size()-1);
        int right = GetLastK(data,k,0,data.size()-1);
//        cout<<left<<" "<<right<<" "<<endl;
        if (left>-1 && right>-1)
        {
        	ans = right - left +1;
            return ans;
		}
		
		else return 0;
        
        
	}
	
	int GetFirstK(vector<int>data,int k,int left,int right)
	{
		
		if (right<left)return -1;
		
		int mid = (left+right)/2;
		if (data[mid]<k)left = mid+1;
		else if (data[mid]>k)right = mid-1;
		else if (data[mid]==k)
		{
			if (mid==0 ||(mid>0 && data[mid-1]!=k))return mid;
			else
				right = mid-1;
		}
		
		return GetFirstK(data,k,left,right);
	}
	
	int GetLastK(vector<int>data,int k,int left,int right)
	{
		if (right<left)return -1;
		
		int l = data.size()-1;
		
		int mid = (left+right)/2;
		if (data[mid]<k)left = mid+1;
		else if (data[mid]>k)right = mid-1;
		else if  (data[mid]==k)
		{
			if (mid==l || (mid<l && data[mid+1]!=k))return mid;
			else left = mid+1;
		}
		return  GetLastK(data,k, left, right);
	}
	
};
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https://www.nowcoder.com/questionTerminal/70610bf967994b22bb1c26f9ae901fa2

    //循环写法
    private int getLastK(int [] array , int k, int start, int end){
        int length = array.length;
        int mid = (start + end) >> 1;
        while(start <= end){
            if(array[mid] > k){
                end = mid-1;
            }else if(array[mid] < k){
                start = mid+1;
            }else if(mid+1 < length && array[mid+1] == k){
                start = mid+1;
            }else{
                return mid;
            }
            mid = (start + end) >> 1;
        }
        return -1;
    }

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某大神利用 stl 的 ：

https://www.nowcoder.com/questionTerminal/70610bf967994b22bb1c26f9ae901fa2

class Solution {
public:
    int GetNumberOfK(vector<int> data ,int k) {
        auto resultPair = equal_range(data.begin(), data.end(),k);
        return resultPair.second - resultPair.first;
    }
};