PostOrderTraversal.java

/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
import java.util.Stack;
public class PostOrderTraversal {
    private class NodeInfo{
        TreeNode node;
        boolean isLeftVisited;
        boolean isRightVisited;
        
        public NodeInfo(TreeNode node){
            this.node = node;
            isLeftVisited = false;
            isRightVisited = false;
            if(node.left == null) isLeftVisited = true;
            if(node.right == null) isRightVisited = true;
        }
    }
    
    public ArrayList<Integer> postorderTraversal(TreeNode root) {
        ArrayList<Integer> result = new ArrayList<Integer>();
        if(root == null) return result;
        
        Stack<NodeInfo> stack = new Stack<NodeInfo>();
        NodeInfo rNode = new NodeInfo(root);
        stack.push(rNode);
        while(stack.size() > 0){
            // peep at the top node
            NodeInfo n = stack.peek();
            // if left side exists and is unvisited, go visit
            if(n.node.left != null && n.isLeftVisited == false){
                n.isLeftVisited = true;
                visitSubTree(new NodeInfo(n.node.left), stack, result);
            }// else if right side exists and is unvisited, go visit
            else if(n.node.right != null && n.isRightVisited == false){
                n.isRightVisited = true;
                visitSubTree(new NodeInfo(n.node.right), stack, result);
            }// else if both sides are visited, pop it
            else if(n.isLeftVisited == true && n.isRightVisited == true){
                result.add(n.node.val);
                stack.pop();
            }
        }
        return result;
    }
    
    private void visitSubTree(NodeInfo n, Stack<NodeInfo> stack, ArrayList<Integer> result){
        while(isLeafNode(n) == false){
            stack.push(n);
            if(n.node.left != null && n.isLeftVisited == false){
                n.isLeftVisited = true;
                n = new NodeInfo(n.node.left);
            }// else if right side exists and is unvisited, go visit
            else if(n.node.right != null && n.isRightVisited == false){
                n.isRightVisited = true;
                n = new NodeInfo(n.node.right);
            }
        }
        // if n.node is a leaf, add it to the result
        result.add(n.node.val);
    }
    
    private boolean isLeafNode(NodeInfo n){
        if(n.node.left == null && n.node.right == null) return true;
        return false;
    }
}