Binary Tree Postorder Traversal

题目介绍

LeetCode 145. Binary Tree Postorder Traversal。

复杂度

时间复杂度: O(n), 空间复杂度: O(n)

解题思路

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 postorderTraversal(_ root: TreeNode?) -> [Int] {
        guard let root = root else {
            return []
        }
        var ans: [Int] = []
        ans += postorderTraversal(root.left)
        ans += postorderTraversal(root.right)
        ans.append(root.val)
        return ans
    }

    func postorderTraversal1(_ root: TreeNode?) -> [Int] {
        guard let root = root else {
            return []
        }
        var ans: [Int] = []
        var stack: [TreeNode] = [root]
        while let node = stack.popLast() {
            ans.append(node.val)
            if let left = node.left {
                stack.append(left)
            }
            if let right = node.right {
                stack.append(right)
            }
        }
        return ans.reversed()
    }
}

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