【c语言】统计一个数字在排序数组中出现的次数

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// 题目：统计一个数字在排序数组中出现的次数。

// 例如：排序数组｛1，2，3，3，3，3，4，5｝和数字3，由于3出现了4次，因此输出4

有一种最简单的算法，遍历。但是有比它效率更高的

先看遍历：

#include <stdio.h>#include <assert.h>int num_time(int *arr, int len, int a){int i = 0;int count = 0;assert(arr != NULL);for (; i < len; ++i){if (arr[i] == a)count++;}return count;}int main(){int arr[] = { 1, 2, 3, 3, 3, 3, 4, 5 };int len = sizeof(arr) / sizeof(arr[0]);printf("%d\n", num_time(arr, len, 3));return 0;}

还有一种利用二分查找：

#include <stdio.h>int GetFirstKey(int arr[], int left, int right, int len, int key){int mid;if (left > right){return -1;}mid = left - (left - right) / 2;if (key == arr[mid]){if ((mid > 0 && arr[mid - 1] != key) || mid == 0){return mid;}else{right = mid - 1;}}else if (arr[mid] < key){left = mid + 1;}else{right = mid - 1;}return GetFirstKey(arr, left, right, len, key);}int GetLastKey(int arr[], int left, int right, int len, int key){int mid;if (left > right){return -1;}mid = left - (left - right) / 2;if (key == arr[mid]){if ((mid < len - 1 && arr[mid + 1] != key || mid == len - 1)){return mid;}else{left = mid + 1;}}else if (arr[mid] < key){left = mid + 1;}else{right = mid - 1;}return GetLastKey(arr, left, right, len, key);}int  main(){int brr[] = { 1, 2, 3, 3, 3, 3, 4, 5};int len = sizeof(brr) / sizeof(brr[0]);int first = GetFirstKey(brr, 0, len - 1, len - 1, 3);int last = GetLastKey(brr, 0, len - 1, len - 1, 3);printf("%d\n", last - first + 1);return 0;}

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