My accepted JAVA solution

• Straightforward thought:
You will meet a node three times from a stack.
For the first time, push the left. The second time, push the right, The third time you meet it, move it to result.

``````class TreeNodeStack {
TreeNode node;
int count;

TreeNodeStack(TreeNode node) {
this.node = node;
}
}

public List<Integer> postorderTraversal(TreeNode root) {
List<Integer> result = new ArrayList<Integer>();
Deque<TreeNodeStack> stack = new ArrayDeque<TreeNodeStack>();
while (stack.size() > 0) {
TreeNodeStack s = stack.peekLast();
s.count++;
if (s.node == null) {
stack.pollLast();
}
else if(s.count==1){
}
else if(s.count==2){
}
else if (s.count == 3) {
stack.pollLast();
}
}
return result;
}``````

• your code is perfect good by using extra ds

• Inspired by peterdyf, thanks!

``````public List<Integer> postorderTraversal(TreeNode root) {
/**
* Write out a recursive version
* Then try to translate it to iterative version.
*
* Translate process :
* When a method is called, we push a function frame into callStack
* After the method finishes, we pop the function frame out.
*
* line1 visit(root.left) if root.left is not null
* line2 visit(root.right) if root.right is not null
* line3 visit(root)
*/

List<Integer> postOrder = new ArrayList<Integer>();

if (root == null) {
return postOrder;
}

Stack<FunctionFrame> callStack = new Stack<FunctionFrame>();
callStack.push(new FunctionFrame(root));

while (!callStack.isEmpty()) {
FunctionFrame curFrame = callStack.peek();

if (curFrame.programCounter == 1) {
if (curFrame.treeNode.left != null) {
callStack.push(new FunctionFrame(curFrame.treeNode.left));
}
} else if (curFrame.programCounter == 2) {
if (curFrame.treeNode.right != null) {
callStack.push(new FunctionFrame(curFrame.treeNode.right));
}
} else if (curFrame.programCounter == 3) {
} else {
callStack.pop();
}

curFrame.programCounter++;
}

return postOrder;
}

public class FunctionFrame {
public TreeNode treeNode;
// programCounter == 1 means visit(root.left) is calling
public int programCounter;

public FunctionFrame(TreeNode treeNode) {
this.treeNode = treeNode;
/**
* When a new function frame is created,
* we move program counter to its first line, meaning first line is calling
*/
this.programCounter = 1;
}
}``````

• You can use Stack instead of Deque, because you don't need to operate at the two ends.

• Thanks. I used to use Stack but Java Docs say Deque interface and its implementations should be used in preference to Stack

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