PostOrder
public class Solution { // Important, when you pop a node, ensure its children are traversed. public ListpostorderTraversal(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 ListpreorderTraversal(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 ListinorderTraversal(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 ListpreorderTraversal(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 ListinorderTraversal(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; } }
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