编程算法

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数字在排序数组中出现的次数 代码(C)

本文地址: http://blog.csdn.net/caroline_wendy

题目: 统计一个数字在排序数组中出现的次数.

通过折半查找, 找到首次出现的位置, 再找到末次出现的位置, 相减就可以.

时间复杂度O(logn).

代码:

/*
 * main.cpp
 *
 *  Created on: 2014.6.12
 *      Author: Spike
 */

/*eclipse cdt, gcc 4.8.1*/

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int GetFirstK (int* data, int length, int k, int start, int end) {
	if (start > end)
		return -1;
	int middleIndex = (start + end)/2;
	int middleData = data[middleIndex];

	if (middleData == k) {
		if ((middleIndex>0 && data[middleIndex-1]!=k) || middleIndex == 0)
			return middleIndex;
		else
			end = middleIndex-1;
	} else if (middleData>k)
		end = middleIndex-1;
	else
		start = middleIndex+1;
	return GetFirstK(data, length, k, start, end);
}

int GetLastK (int* data, int length, int k, int start, int end) {
	if (start > end)
		return -1;
	int middleIndex = (start+end)/2;
	int middleData= data[middleIndex];
	if (middleData == k) {
		if ((middleIndex<length-1 && data[middleIndex+1]!=k) || middleIndex == length-1)
			return middleIndex;
		else
			start = middleIndex+1;
	} else if (middleData < k)
		start = middleIndex+1;
	else
		end = middleIndex-1;
	return GetLastK(data, length, k, start, end);
}

int GetNumberOfK (int* data, int length, int k) {
	int number = 0;
	if (data == NULL && length <= 0)
		return number;
	int first = GetFirstK(data, length, k, 0, length-1);
	int last = GetLastK(data, length, k, 0, length-1);
	if (first > -1 && last > -1)
		number = last-first+1;
	return number;
}

int main(void)
{
	int data[] = {1, 2, 3, 3, 3, 3, 4, 5};
	int k = 3;
	int result = GetNumberOfK(data, 8, k);
	printf("result = %d
", result);

    return 0;
}

输出:

result = 4