lintcode 二分查找

题目：二分查找

描述：给定一个排序的整数数组（升序）和一个要查找的整数target，用O(logn)的时间查找到target第一次出现的下标（从0开始），如果target不存在于数组中，返回-1。

class Solution {
public:
    /**
     * @param nums: The integer array.
     * @param target: Target number to find.
     * @return: The first position of target. Position starts from 0. 
     */
    int binarySearch(vector<int> &array, int target) {
        
        int left = 0;
        int right = array.size() - 1;
        
        while (left + 1 < right) {
            int mid = left + (right - left) / 2;
            if (array[mid] >= target) {
                right = mid;
            } else {
                left = mid;
            }
        }
        
        if (array[left] == target) {
            return left;
        }
        if (array[right] == target) {
            return right;
        }
        
        return -1;
    }
};