统计一个数字在升序数组中出现的次数。

二分法找上下界，注意边界。

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