Binary Search

Question:

Given an interger X and intergers A0，A1，……，A(n-1), which are presorted and already in memory,find i such that Ai=X, or return i=-1 if X is not in the input.

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Soulution:

template <typename T>
int binarySearch(<const vector<T> & a, const T &x) 
{
    int low=0,high=a.size()-1;

    while(low<=high)
    {   
        int mid=(low+high)/2;
        if (a[mid]<x)
            low=mid+1;
        else if (a[mid]>x)
            high = mid-1;
        else return mid;
    }   
    return -1; 
}

 

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Analysis:

采用二分法查找元素。算法复杂度为O(logN)。要求容器内元素已经按升序排列

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Author: Cat