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PostorderTraversal145.java
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PostorderTraversal145.java
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package Leetcode;
import java.util.ArrayList;
import java.util.Deque;
import java.util.LinkedList;
import java.util.List;
public class PostorderTraversal145 {
/**Method 1: Iteration; Time:O(N); Space:O(N) **/
public List<Integer> postorderTraversal(TreeNode root) {
List<Integer> res = new ArrayList<>();
dfs(root, res);
return res;
}
private void dfs(TreeNode node, List<Integer> res) {
if(node == null) {
return;
}
dfs(node.left, res);
dfs(node.right, res);
res.add(node.val);
}
/**Method 2: Not Iteration - Queue; Time:O(N); Space:O(N) **/
public List<Integer> postorderTraversal2(TreeNode root) {
List<Integer> res = new ArrayList<>();
if(root == null) {
return res;
}
Deque<TreeNode> queue = new LinkedList<>();
TreeNode prev = null;
while(!queue.isEmpty() || root != null) {
while(root != null) {
queue.push(root);
root = root.left;
}
root = queue.pop();
if(root.right == null || root.right == prev) {
res.add(root.val);
prev = root;
root = null;
} else {
queue.push(root);
root = root.right;
}
}
return res;
}
//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;
}
}
}