題目：
請實現(xiàn)一個函數(shù)按照之字形打印二叉樹，即第一行按照從左到右的順序打印，第二層按照從右至左的順序打印静秆，第三行按照從左到右的順序打印，其他行以此類推巡李。
思路：
層次遍歷通過一個層數(shù)來控制左-右或者右-左,右到左相當(dāng)于一個reverse所以可以根據(jù)棧的特性來完成這道題.
代碼：
/**
* Created by Hammy on 2018/2/1.
*/
public class Print
{
public ArrayList<ArrayList<Integer> > Print(TreeNode pRoot) {
ArrayList<ArrayList<Integer>> arrayLists = new ArrayList<>();
if(pRoot == null)
return arrayLists;
//代表當(dāng)前層數(shù)
int postNumber = 1 ;
//放奇數(shù)層的棧
Stack<TreeNode> stack1 = new Stack<>();
//偶數(shù)層
Stack<TreeNode> stack2 = new Stack<>();
stack1.push(pRoot);
while(!stack1.isEmpty() || !stack2.isEmpty()){
//代表奇數(shù)
if(postNumber % 2 != 0){
ArrayList<Integer> arrayList = new ArrayList<>();
while(!stack1.isEmpty()){
TreeNode node = stack1.pop();
if(node != null){
arrayList.add(node.val);
stack2.push(node.left);
stack2.push(node.right);
}
}
if(!arrayList.isEmpty()){
arrayLists.add(arrayList);
postNumber++;
}
}else{
ArrayList<Integer> arrayList = new ArrayList<>();
while(!stack2.isEmpty()){
TreeNode node = stack2.pop();
if(node != null){
arrayList.add(node.val);
stack1.push(node.right);
stack1.push(node.left);
}
}
if(!arrayList.isEmpty()){
arrayLists.add(arrayList);
postNumber++;
}
}
}
return arrayLists;
}
}
