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145. Binary Tree Postorder Traversal

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Description

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

 

Example 1:

Input: root = [1,null,2,3]
Output: [3,2,1]

Example 2:

Input: root = []
Output: []

Example 3:

Input: root = [1]
Output: [1]

Example 4:

Input: root = [1,2]
Output: [2,1]

Example 5:

Input: root = [1,null,2]
Output: [2,1]

 

Constraints:

• The number of the nodes in the tree is in the range [0, 100].
• -100 <= Node.val <= 100

 

Follow up:

Recursive solution is trivial, could you do it iteratively?

 

Solutions

Python3

Recursive:

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
class Solution:
    def postorderTraversal(self, root: TreeNode) -> List[int]:
        def postorder(root):
            if root:
                postorder(root.left)
                postorder(root.right)
                res.append(root.val)
        res = []
        postorder(root)
        return res

Non-recursive:

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
class Solution:
    def postorderTraversal(self, root: TreeNode) -> List[int]:
        if not root:
            return []
        s1 = [root]
        s2 = []
        while s1:
            node = s1.pop()
            s2.append(node.val)
            if node.left:
                s1.append(node.left)
            if node.right:
                s1.append(node.right)
        return s2[::-1]

Java

Recursive:

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {

    private List<Integer> res;

    public List<Integer> postorderTraversal(TreeNode root) {
        res = new ArrayList<>();
        postorder(root);
        return res;
    }

    private void postorder(TreeNode root) {
        if (root != null) {
            postorder(root.left);
            postorder(root.right);
            res.add(root.val);
        }
    }
}

Non-recursive:

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    public List<Integer> postorderTraversal(TreeNode root) {
        if (root == null) {
            return Collections.emptyList();
        }
        Deque<TreeNode> s1 = new ArrayDeque<>();
        List<Integer> s2 = new ArrayList<>();
        s1.push(root);
        while (!s1.isEmpty()) {
            TreeNode node = s1.pop();
            s2.add(node.val);
            if (node.left != null) {
                s1.push(node.left);
            }
            if (node.right != null) {
                s1.push(node.right);
            }
        }
        Collections.reverse(s2);
        return s2;
    }
}

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