非递归遍历二叉树

public class Solution {


    public static void main(String[] args) {}

    public List<Integer> preOrderTravel(TreeNode root) {
        List<Integer> result = new ArrayList<>();

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

        Stack<TreeNode> stack = new Stack<>();
        stack.push(root);

        while(!stack.isEmpty()) {
            TreeNode current = stack.pop();

            result.add(current.val);

            if(current.right != null) {
                stack.push(current.right);
            }

            if(current.left != null) {
                stack.push(current.left);
            }
        }

        return result;
    }

    public List<Integer> inOrderTravel(TreeNode root) {
        List<Integer> result = new ArrayList<>();

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

        Stack<TreeNode> stack = new Stack<>();

        TreeNode p = root;

        while(p != null || !stack.isEmpty()) {
            if(p != null) {
                stack.push(p);
                p = p.left;
            } else {
                p = stack.pop();
                result.add(p.val);
                p = p.right;
            }
        }

        return result;
    }

    public static void postOrderTravel(TreeNode root) {

        List<Integer> result = new ArrayList<>();
        if(root == null) {
            return result;
        }

        if(root != null) {
            Stack<TreeNode> stack1 = new Stack<>();
            Stack<TreeNode> stack2 = new Stack<>();

            stack1.push(root);
            while(!stack1.isEmpty()) {
                TreeNode cur = stack1.pop();
                stack2.push(cur);

                if(cur.left != null) {
                    stack1.push(cur.left);
                }

                if(cur.right != null) {
                    stack1.push(cur.right);
                }
            }

            while(!stack2.isEmpty()) {
                //System.out.println(stack2.pop().val);
                result.add(stack2.pop().val);
            }
        }
    }
}