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| import Foundation
extension MyTreeNodeSolution {
//递归
func postOrderTraversal(_ root: TreeNode?,_ n: inout [Int]) {
if root == nil {
return
}
postOrderTraversal(root?.left, &n)
postOrderTraversal(root?.right, &n)
n.append(root!.val)
}
//非递归
//1.左孩子和右孩子都没有，可以访问该节点
//2.存在左孩子或者右孩子，均被访问了，才能访问该节点
//入栈顺序，先右再左。按出栈顺序来
func postOrderTraversal2(_ root: TreeNode?) -> [Int] {
var stack = [TreeNode?]()
var cur: TreeNode?
var pre: TreeNode? = nil
var res = [Int]()
stack.append(root)
while !stack.isEmpty {
cur = stack.last!
if (cur?.left == nil && cur?.right == nil) || (pre != nil && (pre === cur?.left || pre === cur?.right)) {
res.append(stack.removeLast()!.val)
pre = cur
}else {
if cur?.right != nil {
stack.append(cur!.right)
}
if cur?.left != nil {
stack.append(cur!.left)
}
}
}
return res
}
}
|
算法思想：后序遍历：左子树 —> 右子树 —> 根结点
github地址：https://github.com/cubegao/LeetCode
