二叉树非递归遍历

• 前序遍历

  public List<Integer> preorderTraversal(TreeNode root) {
      List<Integer> res = new ArrayList<>();
      Deque<TreeNode> stack = new ArrayDeque<>();
      if(root == null)  return res;
      stack.push(root);
      while(!stack.isEmpty()) {
          TreeNode curr = stack.pop();
          res.add(curr.val);
          if(curr.right != null) stack.push(curr.right);
          if(curr.left != null) stack.push(curr.left);
      }
      return res;
  }
  

• 中序遍历

  public List<Integer> inorderTraversal(TreeNode root) {
      List<Integer> res = new ArrayList<>();
      if(root == null) return res;
      Deque<TreeNode> stack = new ArrayDeque<>();
      while(!stack.isEmpty() || root!=null) {
          while(root != null) {
              stack.push(root);
              root = root.left;
          }
          root = stack.pop();
          res.add(root.val);
          root = root.right;
      }
      return res;
  }
  

• 后序遍历

  public List<Integer> postorderTraversal(TreeNode root) {
      List<Integer> res = new ArrayList<>();
      if (root == null) return res;
      Deque<TreeNode> stack = new ArrayDeque<>();
      TreeNode prev = null;
      while (!stack.isEmpty() || root!=null) {
          while(root != null) {
              stack.push(root);
              root = root.left;
          }
          root = stack.pop();
          if(root.right == null || root.right == prev) {
              res.add(root.val);
              prev = root;
              root = null;
          } else {
              stack.push(root);
              root = root.right;
          }
      }
      return res;
  }