/**

* Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */

前序遍历

class Solution {
    public List<Integer> preorderTraversal(TreeNode root) {
        Stack<TreeNode> stack = new Stack<>();
        List<Integer> ans = new ArrayList<>();
        while(root!=null||!stack.isEmpty()){
            while(root!=null){
                ans.add(root.val);
                stack.push(root);
                root = root.left;
            }
            root = stack.pop().right;
        }
        return ans;
    }
}

中序遍历

class Solution {
    public List<Integer> inorderTraversal(TreeNode root) {
        Stack<TreeNode> stack = new Stack<>();
        List<Integer> ans = new ArrayList<>();
        while(root!=null||!stack.isEmpty()){
            while(root!=null){
                stack.push(root);
                root = root.left;
            }
            root = stack.pop();
            ans.add(root.val);
            root = root.right;
        }
        return ans;
    }
}

后序遍历

class Solution {
    public List<Integer> postorderTraversal(TreeNode root) {
        Stack<TreeNode> stack = new Stack<>();
        List<Integer> ans = new ArrayList<>();
        HashMap<TreeNode, Integer> map = new HashMap<>();
        while(root!=null||!stack.isEmpty()){
            while(root!=null){
                stack.push(root);
                map.put(root, map.getOrDefault(root, 0)+1);
                root = root.left;
            }
            root = stack.pop();
            int flag = map.get(root);
            if(flag<2){
                stack.push(root);
                map.put(root, map.getOrDefault(root, 0)+1);
                root = root.right;
            } 
            else{
                ans.add(root.val);
                root = null;
            }
        }
        return ans;
    }
}