二叉树的遍历

从上往下打印二叉树

从上往下打印出二叉树的每个节点，同层节点从左至右打印。

public class Solution {
    public ArrayList<Integer> PrintFromTopToBottom(TreeNode root) {
        if(root==null){
            return new ArrayList<Integer>();
        }
        ArrayList<Integer> list = new ArrayList<Integer>();
        Queue<TreeNode> queue = new LinkedList<TreeNode>();
        queue.offer(root);
        while(!queue.isEmpty()){
            TreeNode node = queue.poll();
            list.add(node.val);
            if(node.left!=null) queue.offer(node.left);
            if(node.right!=null) queue.offer(node.right);
        }
        return list;
    }
}

按之字形顺序打印二叉树

请实现一个函数按照之字形打印二叉树，即第一行按照从左到右的顺序打印，第二层按照从右至左的顺序打印，第三行按照从左到右的顺序打印，其他行以此类推。

public class Solution {
    public ArrayList<ArrayList<Integer> > Print(TreeNode pRoot) {
         
        ArrayList<ArrayList<Integer>> results = new ArrayList<ArrayList<Integer>>();
        if(pRoot==null) return results;
        Stack<TreeNode> oddStack = new Stack<TreeNode>();
        Stack<TreeNode> evenStack =new Stack<TreeNode>();
        int flag=0;
        oddStack.push(pRoot);
        while(!oddStack.isEmpty()||!evenStack.isEmpty()){
            ArrayList<Integer> list = new ArrayList<Integer>();
            if(flag%2==0){
                while(!oddStack.isEmpty()){
                    TreeNode tNode = oddStack.pop();
                    list.add(tNode.val);
                    if(tNode.left!=null) evenStack.push(tNode.left);
                    if(tNode.right!=null) evenStack.push(tNode.right);
                }
            }else{
                while(!evenStack.isEmpty()){
                    TreeNode tNode = evenStack.pop();
                    list.add(tNode.val);
                    if(tNode.right!=null) oddStack.push(tNode.right);
                    if(tNode.left!=null) oddStack.push(tNode.left);
                }
            }
            flag++;
            results.add(list);
        }
        return results;
    }
 
}