Binary Tree Postorder Traversal

最新推荐文章于 2022-07-19 11:52:33 发布

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本文介绍了一种实现二叉树后序遍历的方法，包括递归和非递归两种实现方式。递归实现简单直观，而非递归实现则通过栈结构来模拟递归过程，避免了递归带来的潜在栈溢出风险。

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Given a binary tree, return the postorder traversal of its nodes' values.

For example:

Given binary tree {1,#,2,3},

return [3,2,1].

递归实现：

public List<Integer> postorderTraversal(TreeNode root) {
	List<Integer> result=new ArrayList<Integer>();
	if(root==null)
		return result;
	result.addAll(postorderTraversal(root.left));
	result.addAll(postorderTraversal(root.right));
	result.add(root.val);
	return result;
}

循环实现：

public List<Integer> postorderTraversal(TreeNode root) {
	ArrayList<Integer> result = new ArrayList<Integer>();
	Stack<TreeNode> stack = new Stack<TreeNode>();
	TreeNode prev = null; // previously traversed node
	TreeNode curr = root;
	if (root == null) 
		return result;
	stack.push(root);
	while (!stack.empty()) {
		curr = stack.peek();
		if (prev == null || prev.left == curr || prev.right == curr) { // traverse down the tree
			if (curr.left != null) 
				stack.push(curr.left);
			else if (curr.right != null)
				stack.push(curr.right);
		} else if (curr.left == prev) { // traverse up the tree from the left
			if (curr.right != null) 
				stack.push(curr.right);
		} else { // traverse up the tree from the right
			result.add(curr.val);
			stack.pop();
		}
		prev = curr;
	}
	return result;
}