[Swift]LeetCode111. 二叉树的最小深度 | Minimum Depth of Binary Tree

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Given a binary tree, find its minimum depth.

The minimum depth is the number of nodes along the shortest path from the root node down to the nearest leaf node.

Note: A leaf is a node with no children.

Example:

Given binary tree [3,9,20,null,null,15,7],

    3
   / \
  9  20
    /  \
   15   7

return its minimum depth = 2.

---

给定一个二叉树，找出其最小深度。

最小深度是从根节点到最近叶子节点的最短路径上的节点数量。

说明: 叶子节点是指没有子节点的节点。

示例:

给定二叉树 [3,9,20,null,null,15,7],

    3
   / \
  9  20
    /  \
   15   7

返回它的最小深度  2.

---

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     public var val: Int
 *     public var left: TreeNode?
 *     public var right: TreeNode?
 *     public init(_ val: Int) {
 *         self.val = val
 *         self.left = nil
 *         self.right = nil
 *     }
 * }
 */
class Solution {
    func minDepth(_ root: TreeNode?) -> Int {
        //深度优先搜索DFS
        if root == nil {return 0}
        if root!.left == nil && root!.right == nil
        {
            return 1
        }
        if root!.left == nil
        {
            return minDepth(root!.right) + 1
        }
        else if root!.right == nil
        {
            return minDepth(root!.left) + 1
        }
        else
        {
            let depthLeft:Int = minDepth(root!.left)
            let depthRight:Int = minDepth(root!.right)
            return (1 + ((depthLeft < depthRight) ? depthLeft : depthRight))
        }
    }
}

---

32ms

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     public var val: Int
 *     public var left: TreeNode?
 *     public var right: TreeNode?
 *     public init(_ val: Int) {
 *         self.val = val
 *         self.left = nil
 *         self.right = nil
 *     }
 * }
 */
class Solution {
    func minDepth(_ root: TreeNode?) -> Int {
        guard let root = root else { return 0 }
        let left = minDepth(root.left)
        let right = minDepth(root.right)
        return 1 + ((left == 0 || right == 0) ? max(left, right) : min(left, right))
    }
}

---

28ms

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     public var val: Int
 *     public var left: TreeNode?
 *     public var right: TreeNode?
 *     public init(_ val: Int) {
 *         self.val = val
 *         self.left = nil
 *         self.right = nil
 *     }
 * }
 */
class Solution {
    func minDepth(_ root: TreeNode?) -> Int {
        guard let root = root else {
            return 0
        }
        
        let left = minDepth(root.left)
        let right = minDepth(root.right)
        
        if left == 0 {
            return right + 1
        } else if right == 0 {
            return left + 1
        } else {
            return min(left, right) + 1
        }
    }
}