问题描述
请实现一个函数按照之字形顺序打印二叉树,即第一行按照从左到右的顺序打印,第二层按照从右到左的顺序打印,第三行再按照从左到右的顺序打印,其他行以此类推。
示例
给定二叉树: [3,9,20,null,null,15,7],
3
/
9 20
/
15 7
返回其层次遍历结果:
[
[3],
[20,9],
[15,7]
]
/**
* Definition for a binary tree node.
* public class TreeNode {
* int val;
* TreeNode left;
* TreeNode right;
* TreeNode(int x) { val = x; }
* }
*/
class Solution {
public List<List<Integer>> levelOrder(TreeNode root) {
if(root==null)
return new ArrayList<List<Integer>>();
List<List<Integer>> res=new ArrayList<List<Integer>>();
//正着先右,倒着先左
List<TreeNode> objective=new ArrayList<TreeNode>();
objective.add(root);
List<Integer> t=new ArrayList<Integer>(1);
t.add(root.val);
res.add(t);
List<TreeNode> negetive=new ArrayList<TreeNode>();
while(objective.size()>0) {
List<Integer> t1=new ArrayList<Integer>();
for(int i=objective.size()-1;i>=0;i--) {
if(objective.get(i).right!=null) {
t1.add(objective.get(i).right.val);
negetive.add(objective.get(i).right);
}
if(objective.get(i).left!=null) {
t1.add(objective.get(i).left.val);
negetive.add(objective.get(i).left);
}
}
if(t1.size()>0)
res.add(t1);
objective.clear();
if(negetive.size()>0) {
List<Integer> t2=new ArrayList<Integer>();
for(int i=negetive.size()-1;i>=0;i--) {
if(negetive.get(i).left!=null) {
t2.add(negetive.get(i).left.val);
objective.add(negetive.get(i).left);
}
if(negetive.get(i).right!=null) {
t2.add(negetive.get(i).right.val);
objective.add(negetive.get(i).right);
}
}
if(t2.size()>0)
res.add(t2);
negetive.clear();
}
}
return res;
}
}