PostOrder

public class Solution {    // Important, when you pop a node, ensure its children are traversed.    public List
    
      postorderTraversal(TreeNode root) {        ArrayDeque
     
       s = new ArrayDeque();        List
      
        ans = new ArrayList
       
        ();        TreeNode cur = root;                while (cur != null || !s.isEmpty()) {            while (cur != null) {                s.push(cur);                cur = cur.left;            }                        while (!s.isEmpty() && cur == s.peek().right) {                cur = s.pop();                ans.add(cur.val);            }                        if (s.isEmpty()) cur = null; else cur = s.peek().right;        }                return ans;    }    }
       
      
     
    

PreOrder

public class Solution {    public List
    
      preorderTraversal(TreeNode root) {        List
     
       res = new LinkedList
      
       ();        ArrayDeque
       
         stk = new ArrayDeque
        
         ();        TreeNode cur = root;                while(cur != null || !stk.isEmpty()){            if(cur != null) {                stk.push(cur);                res.add(cur.val);   // add val before going to children                cur = cur.left;            } else {                TreeNode node = stk.pop();                cur = node.right;            }        }        return res;    }}
        
       
      
     
    

InOrder

public class Solution {    public List
    
      inorderTraversal(TreeNode root) {        List
     
       res = new ArrayList
      
       ();        if(root == null) return res;        ArrayDeque
       
         stk = new ArrayDeque
        
         ();        TreeNode cur = root;                while(cur != null || !stk.isEmpty()){            if(cur !=null) {                stk.push(cur);                cur = cur.left;             } else {                TreeNode node = stk.pop();                res.add(node.val);        // add after all left children                node = node.right;            }        }        return res;    }}
        
       
      
     
    

PreOrder

public class Solution {    public List
    
      preorderTraversal(TreeNode root) {        List
     
       res = new LinkedList
      
       ();        ArrayDeque
       
         stk = new ArrayDeque
        
         ();        if(root == null) return res;        TreeNode cur = root;        stk.push(cur);                while(!stk.isEmpty()){            cur = stk.pop();            res.add(cur.val);            if(cur.right != null) stk.push(cur.right);            if(cur.left != null) stk.push(cur.left);        }        return res;    }}
        
       
      
     
    

InOrder

public class Solution {    public List
    
      inorderTraversal(TreeNode root) {        List
     
       res = new ArrayList
      
       ();        if(root == null) return res;        ArrayDeque
       
         stk = new ArrayDeque
        
         ();        TreeNode cur = root;                while(cur != null || !stk.isEmpty()){            while(cur != null) {                stk.push(cur);                cur = cur.left;            }            cur = stk.pop();            res.add(cur.val);            cur = cur.right;        }        return res;    }}