Your algorithm's runtime complexity must be in the order of O(log n).
If the target is not found in the array, return [-1, -1].
For example,
Given [5, 7, 7, 8, 8, 10] and target value 8,
return [3, 4].
这道题目同样采用二分法,当找到目标元素后,因为存在重复元素,我们要从当前元素向两边扩散,然后得到一个范围。如何记录呢,我们可以new一个包含两个元素的数组result,当找到目标元素时,记录当前元素的下标m,让result[0] = m, result[1] = m, 然后以m为中心,向两边比较,左边相等的就让result[0]减1,右边相等的就让result[1]加1。最终得到了包含目标元素的范围。代码如下:
public class Solution { public int[] searchRange(int[] nums, int target) { int[] result = new int[2]; result[0] = -1; result[1] = -1; if(nums == null || target < nums[0] || target > nums[nums.length - 1]) return result; int l = 0; int r = nums.length - 1; while(l <= r) { int m = l + (r - l) / 2; if(nums[m] == target) { result[0] = m; result[1] = m; while(result[0] > 0 && nums[result[0] - 1] == target) result[0] --; while(result[1] < nums.length - 1 && nums[result[1] + 1] == target) result[1] ++; return result; } else if(nums[m] > target) { r = m - 1; } else { l = m + 1; } } return result; } }