《剑指offer》- 统计一个数字在排序数组中出现的次数

/*
 * 统计一个数字在排序数组中出现的次数。
 * 注：数组已有序
 */
public class GetNumberOfK {
	//算法复杂度：O(n)
	public int getNumberOfK(int [] array , int k) {
		int count = 0;
		for(int i : array) {
			if(k == i)	count++;
		}
		
		return count;
    }
	
	//运用二分法查找该数第一次出现的位置和最后一次出现的位置，算法复杂度：getFirstIndex和getLastIndex的时间复杂度都是O(logn)
	public int getNumberOfK2(int [] array , int k) {
		if(array == null || array.length == 0)	return 0;
		
		int firstIndex = getFirstIndex(array, k, 0, array.length - 1);
		int lastIndex = getLastIndex(array, k, 0, array.length - 1);
		if(firstIndex != -1 && lastIndex != -1) {
			return lastIndex - firstIndex + 1;
		}
		
		return 0;
		
    }
	
	public int getFirstIndex(int [] array, int k, int start, int end) {
		if(start > end)
			return -1;
		
		int middle = (start + end) / 2;
		if(array[middle] == k) {
			if(middle == 0 || (middle - 1 >= 0 && array[middle - 1] != k)) {
				return middle;
			}
			else {
				end = middle - 1;
			}
		}
		else if(array[middle] < k) {
			start = middle + 1;
		}
		else {
			end = middle - 1;
		}
		
		return getFirstIndex(array, k, start, end);
		
	}
	
	public int getLastIndex(int [] array, int k, int start, int end) {
		if(start > end)
			return -1;
		
		int middle = (start + end) / 2;
		if(array[middle] == k) {
			if(middle == array.length - 1 || (middle + 1 <= array.length - 1 && array[middle + 1] != k)) {
				return middle;
			}
			else {
				start = middle + 1;
			}
		}
		else if(array[middle] < k) {
			start = middle + 1;
		}
		else {
			end = middle - 1;
		}
		
		return getLastIndex(array, k, start, end);
	}
	
	//法三：借助字符串统计数字第一次出现的位置和最后一次出现的位置
	public int getNumberOfK3(int [] array , int k) {
		if(array == null || array.length == 0)	return 0;
			
		StringBuffer sb = new StringBuffer();
		//System.out.println(sb);
		for(int i : array) {
			sb.append(i);
		}
		int firstIndex = sb.indexOf(k + "");
		int lastIndex = sb.lastIndexOf(k + "");
		//System.out.println("firstIndex:" + firstIndex + " lastIndex:" + lastIndex);
		
		if(firstIndex < 0 || lastIndex < 0)	//不存在这个数字
			return 0;
		return lastIndex - firstIndex + 1;
	}
	
	public static void main(String[] args) {
		int [] array = {1,3,3,3,3,4,5};
		GetNumberOfK t = new GetNumberOfK();
		//System.out.println(t.getLastIndex(array, 3, 0, array.length - 1));
		System.out.println(new GetNumberOfK().getNumberOfK3(array, 2));
	}
}