Preorder, Inorder, and Postorder非递归总结

Preorder, Inorder, and Postorder Iteratively Summarization[1]

1.Pre Order Traverse

 public List<Integer> preorderTraversal(TreeNode root) {
     List<Integer> result = new ArrayList<>();
     Deque<TreeNode> stack = new ArrayDeque<>();
     TreeNode p = root;
     while(!stack.isEmpty() || p != null) {
         if(p != null) {
             stack.push(p);
             result.add(p.val);  // Add before going to children
             p = p.left;
         } else {
             TreeNode node = stack.pop();
             p = node.right;
         }
     }
     return result;
 }

2.In Order Traverse

 public List<Integer> inorderTraversal(TreeNode root) {
     List<Integer> result = new ArrayList<>();
     Deque<TreeNode> stack = new ArrayDeque<>();
     TreeNode p = root;
     while(!stack.isEmpty() || p != null) {
         if(p != null) {
             stack.push(p);
             p = p.left;
         } else {
             TreeNode node = stack.pop();
             result.add(node.val);  // Add after all left children
             p = node.right;
         }
     }
     return result;
 }

3.Post Order Traverse

 public List<Integer> postorderTraversal(TreeNode root) {
     LinkedList<Integer> result = new LinkedList<>();
     Deque<TreeNode> stack = new ArrayDeque<>();
     TreeNode p = root;
     while(!stack.isEmpty() || p != null) {
         if(p != null) {
             stack.push(p);
             result.addFirst(p.val);  // Reverse the process of preorder
             p = p.right;             // Reverse the process of preorder
         } else {
             TreeNode node = stack.pop();
             p = node.left;           // Reverse the process of preorder
         }
     }
     return result;
 }