【剑指offer】数字在排序数组中出现的次数

题目描述

统计一个数字在排序数组中出现的次数。

题解

数组有序，采用二分查找法。

首先找到数字第一个出现的位置，然后找数字最后出现的位置，即可得出数字的个数。

在找数字最后出现的位置时，最后注意临界条件的判断，谨防死循环。

public class Solution {
	
    public int GetNumberOfK(int [] a , int k) {
        if(a == null || a.length == 0)
            return 0;
        int leftIndex = getFirstIndex(a, k, 0, a.length - 1);
    	if(leftIndex == -1)
    		return 0;
        int rightIndex = getLastIndex(a, k, 0, a.length - 1);
    	return rightIndex - leftIndex + 1;
    }
    
    public int getFirstIndex(int[] a, int k, int low, int high) {
        int leftIndex = -1;
        int mid;
    	while(low <= high) {
    		mid = (low + high)/2;
    		if(a[mid] > k)
    			high = mid - 1;
    		else if(a[mid] < k)
    			low = mid + 1;
    		else {
    			if(mid == low) {
    				leftIndex = mid;
    				break;
    			} else {
    				high = mid;
    			}
    		}
    	}
        return leftIndex;
    }
    
    public int getLastIndex(int[] a, int k, int low, int high) {
    	int rightIndex = -1;
        int mid;
    	while(low <= high) {
    		mid = (low + high)/2;
    		if(a[mid] > k)
    			high = mid - 1;
    		else if(a[mid] < k)
    			low = mid + 1;
    		else {
    			if(mid == low) {
    				if(a[high] == k)
    					rightIndex = high;
    				else
    					rightIndex = mid;
    				break;
    			} else {
    				low = mid;
    			}
    		}
    	}
        return rightIndex;
    }
}