[LeetCode] 34. Find First and Last Position of Element in Sorted Array

Find the elements in an ordered array, the first and last position. Meaning of the questions is very simple, to an array A and a digital, ask where to position A digital first and last appearance in the array in, if not, return -1. example,

Example 1:

Input: nums = [5,7,7,8,8,10], target = 8

Output: [3,4]

Example 2:

Input: nums = [5,7,7,8,8,10], target = 6

Output: [-1,-1]

Is a typical dichotomy problem, the idea is to find the first insertion position and the second position by the insertion of ideological dichotomy respectively.

Time O (log n)

Space O (1)

 1 /**
 2  * @param {number[]} nums
 3  * @param {number} target
 4  * @return {number[]}
 5  */
 6 var searchRange = function(nums, target) {
 7     // corner case
 8     if (nums === null || nums.length === 0) {
 9         return [-1, -1];
10     }
11     // normal case
12     let start = findFirst(nums, target);
13     if (start === -1) {
14         return [-1, -1];
15     }
16     let end = findLast(nums, target);
17     return [start, end];
18 };
19 
20 var findFirst = function(nums, target) {
21     let start = 0;
22     let end = nums.length - 1;
23     while (start + 1 < end) {
24         let mid = Math.floor(start + (end - start) / 2);
25         if (nums[mid] < target) {
26             start = mid;
27         } else {
28             end = mid;
29         }
30     }
31     if (nums[start] === target) return start;
32     if (nums[end] === target) return end;
33     return -1;
34 };
35 
36 var findLast = function(nums, target) {
37     let start = 0;
38     let end = nums.length - 1;
39     while (start + 1 < end) {
40         let mid = Math.floor(start + (end - start) / 2);
41         if (nums[mid] > target) {
42             end = mid;
43         } else {
44             start = mid;
45         }
46     }
47     if (nums[end] === target) return end;
48     if (nums[start] === target) return start;
49     return -1;
50 };