[Swift]LeetCode754. 到達終點數字 | Reach a Number

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You are standing at position 0 on an infinite number line. There is a goal at position target.

On each move, you can either go left or right. During the n-th move (starting from 1), you take n steps.

Return the minimum number of steps required to reach the destination.

Example 1:

Input: target = 3
Output: 2
Explanation:
On the first move we step from 0 to 1.
On the second step we step from 1 to 3. 

Example 2:

Input: target = 2
Output: 3
Explanation:
On the first move we step from 0 to 1.
On the second move we step  from 1 to -1.
On the third move we step from -1 to 2. 

Note:

• target will be a non-zero integer in the range [-10^9, 10^9].

---

在一根無限長的數軸上，你站在0的位置。終點在target的位置。

每次你能夠選擇向左或向右移動。第 n 次移動（從 1 開始），能夠走 n 步。

返回到達終點須要的最小移動次數。

示例 1:

輸入: target = 3
輸出: 2
解釋:
第一次移動，從 0 到 1 。
第二次移動，從 1 到 3 。

示例 2:

輸入: target = 2
輸出: 3
解釋:
第一次移動，從 0 到 1 。
第二次移動，從 1 到 -1 。
第三次移動，從 -1 到 2 。

注意:

• target是在[-10^9, 10^9]範圍中的非零整數。

---

Runtime: 4 ms

Memory Usage: 18.4 MB

 1 class Solution {
 2     func reachNumber(_ target: Int) -> Int {
 3         let target = abs(target)
 4         let a: Double = 1; let b: Double = 1; let c: Double = -2 * Double(target);
 5         let delt = b * b + 4 * a * c
 6         //一元二次方程求根公式
 7         var count = Int(ceil((-b+sqrt(abs(delt)))/(2*a)))
 8         // 第n步往回走, 累加值相應減小2n
 9         // 當累加值減去target等於2n時, 返回結果
10         var sum = Int((1 + Double(count)) / 2 * Double(count))
11         while (Int(sum) - target) % 2 != 0 {
12             count += 1
13             sum += count
14         }
15         return count
16     }
17 }

---

Runtime: 8 ms

Memory Usage: 18.6 MB

 1 class Solution {
 2     func reachNumber(_ target: Int) -> Int {
 3         var target = abs(target)
 4         var res:Int = 0
 5         var sum:Int = 0
 6         while (sum < target || (sum - target) % 2 == 1)
 7         {
 8             res += 1
 9             sum += res
10         }
11         return res
12     }
13 }

---

12ms

 1 class Solution {
 2     func reachNumber(_ target: Int) -> Int {
 3         var target = abs(target)
 4         var k = 0
 5         
 6         while (target > 0) {
 7             k += 1
 8             target -= k
 9         }
10         
11         return target&1 == 0 ? k : (k+1+(k&1))
12     }
13 }

---

16ms

 1 class Solution {
 2   func reachNumber(_ target: Int) -> Int {
 3     let target = abs(target)
 4     
 5     var sum = 0
 6     var steps = 0
 7     
 8     while sum < target {
 9       steps += 1
10       sum += steps
11     }
12     
13     let dist = sum - target
14     
15     if dist == 0 || dist % 2 == 0 {
16       return steps
17     } else {
18       if (dist + steps + 1) % 2 == 0 {
19         return steps + 1
20       } else {
21         return steps + 2
22       }
23     }
24   }
25 }