[Swift]LeetCode754. 到达终点数字 | 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].

---

在一根无限长的数轴上，你站在0的位置。终点在target的位置。

每次你可以选择向左或向右移动。第 n 次移动（从 1 开始），可以走 n 步。

返回到达终点需要的最小移动次数。

示例 1:

输入: target = 3
输出: 2
解释:
第一次移动，从 0 到 1 。
第二次移动，从 1 到 3 。

示例 2:

输入: target = 2
输出: 3
解释:
第一次移动，从 0 到 1 。
第二次移动，从 1 到 -1 。
第三次移动，从 -1 到 2 。

注意:

• target是在[-10^9, 10^9]范围中的非零整数。

---

Runtime: 4 ms

Memory Usage: 18.4 MB

class Solution {
    func reachNumber(_ target: Int) -> Int {
        let target = abs(target)
        let a: Double = 1; let b: Double = 1; let c: Double = -2 * Double(target);
        let delt = b * b + 4 * a * c
        //一元二次方程求根公式
        var count = Int(ceil((-b+sqrt(abs(delt)))/(2*a)))
        // 第n步往回走, 累加值相应减少2n
        // 当累加值减去target等于2n时, 返回结果
        var sum = Int((1 + Double(count)) / 2 * Double(count))
        while (Int(sum) - target) % 2 != 0 {
            count += 1
            sum += count
        }
        return count
    }
}

---

Runtime: 8 ms

Memory Usage: 18.6 MB

class Solution {
    func reachNumber(_ target: Int) -> Int {
        var target = abs(target)
        var res:Int = 0
        var sum:Int = 0
        while (sum < target || (sum - target) % 2 == 1)
        {
            res += 1
            sum += res
        }
        return res
    }
}

---

12ms

class Solution {
    func reachNumber(_ target: Int) -> Int {
        var target = abs(target)
        var k = 0
        
        while (target > 0) {
            k += 1
            target -= k
        }
        
        return target&1 == 0 ? k : (k+1+(k&1))
    }
}

---

16ms

class Solution {
  func reachNumber(_ target: Int) -> Int {
    let target = abs(target)
    
    var sum = 0
    var steps = 0
    
    while sum < target {
      steps += 1
      sum += steps
    }
    
    let dist = sum - target
    
    if dist == 0 || dist % 2 == 0 {
      return steps
    } else {
      if (dist + steps + 1) % 2 == 0 {
        return steps + 1
      } else {
        return steps + 2
      }
    }
  }
}