這題跟144和94一樣了贮竟，但這題是hard。遞歸寫法都是一致的固逗，這里貼一下迭代寫法左权，摘自https://discuss.leetcode.com/topic/30632/preorder-inorder-and-postorder-iteratively-summarization/2
很多人說他在cheating，我模擬了一下增拥，好像沒什么問題啄巧，不深究了。
//Post Order Traverse
public List<Integer> postorderTraversal(TreeNode root) {
LinkedList<Integer> result = new LinkedList<>();
Deque<TreeNode> stack = new ArrayDeque<>();
TreeNode p = root;
while(!stack.isEmpty() || p != null) {
if(p != null) {
stack.push(p);
result.addFirst(p.val); // Reverse the process of preorder
p = p.right; // Reverse the process of preorder
} else {
TreeNode node = stack.pop();
p = node.left; // Reverse the process of preorder
}
}
return result;
}
跟preorder對比：
public List<Integer> preorderTraversal(TreeNode root) {
List<Integer> result = new ArrayList<>();
Deque<TreeNode> stack = new ArrayDeque<>();
TreeNode p = root;
while(!stack.isEmpty() || p != null) {
if(p != null) {
stack.push(p);
result.add(p.val); // Add before going to children
p = p.left;
} else {
TreeNode node = stack.pop();
p = node.right;
}
}
return result;
}
