-
Notifications
You must be signed in to change notification settings - Fork 0
Expand file tree
/
Copy pathfirstPositionOfTarget.js
More file actions
54 lines (43 loc) · 1.4 KB
/
Copy pathfirstPositionOfTarget.js
File metadata and controls
54 lines (43 loc) · 1.4 KB
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
// 14. First Position of Target
// Description
// For a given sorted array(ascending order) and a target number, find the first index of this number in O(log n) time complexity.
// If the target number does not exist in the array, return -1.
// Example
// Example 1:
// Input: [1, 4, 4, 5, 7, 7, 8, 9, 9, 10]，1
// Output: 0
// Explanation:
// the first index of 1 is 0.
// Example 2:
// Input: [1, 2, 3, 3, 4, 5, 10]，3
// Output: 2
// Explanation:
// the first index of 3 is 2.
// Example 3:
// Input: [1, 2, 3, 3, 4, 5, 10]，6
// Output: -1
// Explanation:
// Not exist 6 in array.
/**
* @param nums: The integer array.
* @param target: Target to find.
* @return: The first position of target. Position starts from 0.
*/
const binarySearch = function (nums, target) {
// write your code here
let start = 0
let end = nums.length - 1
let mid
while (start + 1 < end) {
mid = start + Math.floor((end - start) / 2)
if (nums[mid] == target) end = mid // !
if (nums[mid] < target) start = mid
if (nums[mid] > target) end = mid
}
if (nums[start] == target) return start
if (nums[end] == target) return end
return -1
}
// tip:第一个 或 最后一个 的问题
// 1 while 循环内 当 if (nums[mid] == target) end = mid
// 这里不能直接返回 mid 而是要缩小范围。可能第一刀切到的 mid 就是数组中第二个符合的数。当要求第一个数时，就错了