题目:统计一个数字在排序数组中出现的次数。例如输入排序数组{1, 2, 3, 3, 3, 3, 4, 5}和数字3,由于3在这个数组中出现了4此,因此输出4。
思路:因为输入的数组为排序的,第一反应是二分查找。先二分查找k第一次出现的位置,再二分查找k最后出现的位置,进而位置相减+1。
public int getNumK(int[] nums, int k) {
int first = helpFirstK(nums, k);
int last = helpLastK(nums, k);
// not found, return -1.
if (first==-1 || last==-1)
return -1;
return (last-first+1);
}
// find the first position of param k
public int helpFirstK(int[] nums, int k) {
int s = 0, e = nums.length-1;
while (s < e-1) {
int mid = (s + e) / 2;
if (nums[mid] >= k) {
e = mid;
} else {
s = mid;
}
}
if (nums[s] == k)
return s;
if (nums[e] == k)
return e;
return -1;
}
// find the last position of param k
public int helpLastK(int[] nums, int k) {
int s = 0, e = nums.length-1;
while (s < e-1) {
int mid = (s + e) / 2;
if (nums[mid] <= k) {
s = mid;
} else {
e = mid;
}
}
if (nums[e] == k)
return e;
if (nums[s] == k)
return s;
return -1;
}