Given a binary tree, return the postorder traversal of its nodes' values.
Example:
Input:[1,null,2,3]
1 \ 2 / 3 Output:[3,2,1]
Follow up: Recursive solution is trivial, could you do it iteratively?
題解如下:
/**
* Definition for a binary tree node.
* public class TreeNode {
* int val;
* TreeNode left;
* TreeNode right;
* TreeNode(int x) { val = x; }
* }
*/
class Solution {
public List<Integer> postorderTraversal(TreeNode root) {
LinkedList<Integer> res = new LinkedList<>();
if(root == null)
return res;
Deque<TreeNode> stack = new ArrayDeque<>();
TreeNode p = root;
while(!stack.isEmpty() || p != null) {
if(p != null) {
stack.push(p);
res.addFirst(p.val);
p = p.right;
} else {
p = stack.pop().left;
}
}
return res;
}
}