剑指offer_数字在排序数组中出现的次数

题目描述

解题思路

lastK - firstK + 1

Java实现

public class Solution {
public int GetNumberOfK(int [] array , int k) {
int length = array.length;
if (length == 0)
return 0;
int firstK = getFirstK(array, k, 0, length-1);
int lastK = getLastK(array, k, 0, length-1);
if (firstK > -1 && lastK > -1)
return lastK - firstK + 1;
return 0;
}

private int getFirstK(int[] array, int k, int start, int end){
int length = array.length;
int mid = start + (end - start) >> 1;
while(start <= end){
if(array[mid] > k){
end = mid - 1;
}else if(array[mid] < k){
start = mid + 1;
}else if(mid-1 >= 0 && array[mid-1] == k){
end = mid - 1;
}else{
return mid;
}
mid = start + ((end - start) >> 1);
}
return -1;
}

private int getLastK(int[] array, int k, int start, int end){
int length = array.length;
int mid = start + (end - start) >> 1;
while(start <= end){
if(array[mid] > k){
end = mid - 1;
}else if(array[mid] < k){
start = mid + 1;
}else if(mid+1 <= end && array[mid+1] == k){
//二分查找改进地方
start = mid + 1;
}else{
return mid;
}
//更新mid，计算mid要使用如下方法
mid = start + ((end - start) >> 1);
}
return -1;
}
}