[LeetCode 145] Binary Tree Postorder Traversal

문제 바로가기

Description

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

Example 1

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

Example 2

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• Input: root = [1,2,3,4,5,null,8,null,null,6,7,9]
• Output: [4,6,7,5,2,9,8,3,1]
• Explanation:
  • 

Example 3

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• Input: root = []
• Output: []

Example 4

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• Input: root = [1]
• Output: [1]

Constraints

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

Code

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내 제출

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/**
 * 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> ans = new ArrayList<>();

    public List<Integer> postorderTraversal(TreeNode root) {
        dfs(root);
        return ans;
    }

    private void dfs(TreeNode root) {
        if (root == null) {
            return;
        }
        dfs(root.left);
        dfs(root.right);
        ans.add(root.val);
    }
}

Runtime	Memory
0 ms	41.8 MB

다른 풀이

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 /**
 * 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) {
        List<Integer> res=new ArrayList<>();
        postorder(root,res);
        return res;
    }
    private void postorder(TreeNode root,List<Integer> res){
        if(root==null){
            return;
        }
        postorder(root.left,res);
        postorder(root.right,res);
        res.add(root.val);

    }
}*/
class Solution{
    public List<Integer> postorderTraversal(TreeNode root){
        List<Integer> ans = new ArrayList<>();
        Stack<TreeNode> st = new Stack<>();
        if(root==null){
            return ans;
        }
        st.push(root);
        while(!st.isEmpty()){
            TreeNode s = st.pop();
            ans.add(s.val);
            if(s.left!=null){
                st.push(s.left);
            }
            if(s.right!=null){
                st.push(s.right);
            }
        }
        Collections.reverse(ans);

        return ans;
    }
}

Reference

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• https://github.com/doocs/leetcode/blob/main/solution/0100-0199/0145.Binary%20Tree%20Postorder%20Traversal/Solution.java