树中常用知识点集合 ---------- 持续更新

 1     //前序遍历的顺序节点存放在list中
 2     public List<TreeNode> preorderUsingStack(TreeNode root) {
 3         List<TreeNode> list = new LinkedList<TreeNode>();
 4         if (root == null) {
 5             return list;
 6         }
 7         Stack<TreeNode> stack = new Stack<TreeNode>();
 8         stack.push(root);
 9         while (!stack.isEmpty()) {
10             TreeNode tempNode = stack.pop();
11             //先存入当前节点
12             list.add(tempNode);
13             //注意是先压栈右子节点再压栈左子节点
14             if (tempNode.right != null) {
15                 stack.push(tempNode.right);
16             }
17             if (tempNode.left != null) {
18                 stack.push(tempNode.left);
19             }
20         }
21         return list;
22     }