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  • 二分查找及其变种简单易懂的模版

    鉴于最近在网上看到的二分查找算法非常复杂,细节太多,不容易理解,下面给出几个实现简洁,又容易理解的代码模版。

    首先,让我们记住最基本的二分查找模版:

    在有序数组A中查找key,如果找到,返回位置索引,否则,返回-1;

    int BinarySearch(int A[], int n, int key)
    {
        int left = 0, right = n - 1;
        while (left <= right) {
            int mid = left + (right - left) / 2;
            if (A[mid] < key) {
                left = mid + 1;
            } else if (A[mid] > key) {
                right = mid - 1;
            } else {// A[mid] == key
                return mid;
            }
        }
        return -1;
    } 

    变种1:如果A有多个key元素,返回最大的,否则,返回-1

    int BinarySearch(int A[], int n, int key)
    {
        int left = 0, right = n - 1;
        int result = -1;
        while (left <= right) {
            int mid = left + (right - left) / 2;
            if (A[mid] < key) {
                left = mid + 1;
            } else if (A[mid] > key) {
                right = mid - 1;
            } else {// A[mid] == key
                result = mid;
                left = mid + 1;//继续查找右半边
            }
        }
        return result;
    } 

    变种2:如果A中有多个key元素,返回最小的,否则,返回-1

    int BinarySearch(int A[], int n, int key)
    {
        int left = 0, right = n - 1;
        int result = -1;
        while (left <= right) {
            int mid = left + (right - left) / 2;
            if (A[mid] < key) {
                left = mid + 1;
            } else if (A[mid] > key) {
                right = mid - 1;
            } else {// A[mid] == key
                result = mid;
                right = mid - 1;// 继续查找左半边
            }
        }
        return result;
    } 

     变种3:在有序数组A中,查找出最小的比key大的元素索引。

    int BinarySearch(int A[], int n, int key)
    {
       if (A[n-1] < key)
            return -1;
        int left = 0, right = n - 1;
        while (left != right) {
            int mid = left + (right - left) / 2;
            if (A[mid] <= key) {
                left = mid + 1;
            } else {
                right = mid;
            } 
        }
        return left;
    }   

    变种4:在有序数组A中,查找出最大的比k小的元素索引。

    int BinarySearch(int A[], int n, int key)
    {
        if (A[0] > key)
            return -1;
        int left = 0, right = n - 1;
        while (left < right-1) {
            int mid = left + (right - left) / 2;
            if (A[mid] >= key) {
                right = mid - 1;
            } else {
                left = mid;
            } 
        }
        if (A[right] < key) {
            return right;
        } else {
            return left;
        }
    } 
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  • 原文地址:https://www.cnblogs.com/xingzhg/p/3942221.html
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