在排序数组中，找出给定数字的出现次数

在排序数组中，找出给定数字的出现次数，比如 [1, 2, 2, 2, 3] 中2的出现次数是3次。

算法：借鉴了stl里面的equal_range算法，关键是处理好边界，二分问题

#include <iostream>  
#include <vector>  
#include <fstream>  
#include <ctime>  
using namespace std;  

int low(vector<int> &ivec, int left, int right, int x)  
{  
    while(left < right)  
    {  
        int mid = (left + right) / 2;  
        if(ivec[mid] < x)  
            left = mid + 1;  
        else  
            right = mid;  
    }  
    return left;  
}  

int upper(vector<int> &ivec, int left, int right, int x)  
{  
    while(left < right)  
    {  
        int mid = (left + right) / 2;  
        if(ivec[mid] > x)  
            right = mid;  
        else  
            left = mid + 1;  
    }  
    return right;  
}  

int main()  
{  
    clock_t begin = clock();  
    int n, x, left, right, first = 0, last = 0;  
    vector<int> ivec;  
    ifstream infile("input2.txt");  
    while(infile >> n)  
        ivec.push_back(n);  
    cout << "Input x: " << endl;  
    cin >> x;  
    left = 0;  
    right = ivec.size();  
    while(left < right)  
    {  
        int mid = (left + right) / 2;  
        if(ivec[mid]  < x)  
            left = mid + 1;  
        else if(ivec[mid] > x)  
            right = mid + 1;  
        else  
        {  
            first = low(ivec, left, right, x);  
            cout << first << endl;  
            last = upper(ivec, left, right, x);  
            break;  
        }  
    }  
    cout << "Numbers: " << last - first << endl;  
    clock_t endtime = clock();  
    cout << "Time: " << (double)(endtime - begin) << endl;  
}