給定一個(gè)二叉樹(shù)柠新，返回它的 后序遍歷。
示例:
輸入: [1,null,2,3]
1
\
2
/
3
輸出: [3,2,1]
進(jìn)階: 遞歸算法很簡(jiǎn)單辉巡，你可以通過(guò)迭代算法完成嗎登颓？
/**
* 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) {
List<Integer> res = new ArrayList<Integer>();
if (root == null) {
return res;
}
Stack<TreeNode> stack = new Stack<TreeNode>();
TreeNode prev = null;
while (root != null || !stack.isEmpty()) {
while (root != null) {
stack.push(root);
root = root.left;
}
root = stack.pop();
if (root.right == null || root.right == prev) {
res.add(root.val);
prev = root;
root = null;
} else {
stack.push(root);
root = root.right;
}
}
return res;
}
}
