二叉树的遍历（循环方式）

前序遍历（Preorder Traverse）

根结点-左子树-右子树

Java代码实现：

public List<Integer> preorderTraversal(TreeNode root) {
        List<Integer> ret = new ArrayList<Integer>();
        if(root == null)
            return ret;
        Stack<TreeNode> stack = new Stack<TreeNode>();
        TreeNode cur = root;
        while(cur != null || !stack.isEmpty()){
            if(cur != null){
                ret.add(cur.val);
                stack.push(cur);
                cur = cur.left;
            }else{
                TreeNode node = stack.pop();
                cur = node.right;
            }
        }
        return ret;
    }

后序遍历（postorder traverse)

左子树-右子树-根结点

代码实现：

public List<Integer> postorderTraversal(TreeNode root) {
        LinkedList<Integer> ret = new LinkedList<Integer>();
        if(root == null)
            return ret;
        Stack<TreeNode> stack = new Stack<TreeNode>();
        TreeNode cur = root;
        while(cur != null || !stack.isEmpty()){
            if(cur != null){
                stack.push(cur);
                ret.addFirst(cur.val);
                cur = cur.right;
            }else{
                TreeNode node = stack.pop();
                cur = node.left;
            }
        }
        return ret;
    }

中序遍历（inorder traverse):

左子树-根结点-右子树

代码实现：

 public List<Integer> inorderTraversal(TreeNode root) {
        List<Integer> ret = new ArrayList<Integer>();
        if(root == null)
            return ret;
        Stack<TreeNode> stack = new Stack<TreeNode>();
        TreeNode cur = root;
        while(!stack.isEmpty() || cur != null){
            if(cur != null){
                stack.push(cur);
                cur = cur.left;
            }else{
                TreeNode node = stack.pop();
                ret.add(node.val);
                cur = node.right;
            }
        }
        return ret;
    }