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// 103. 二叉树的锯齿形层序遍历
/**
* Definition for a binary tree node.
* public class TreeNode {
* int val;
* TreeNode left;
* TreeNode right;
* TreeNode() {}
* TreeNode(int val) { this.val = val; }
* TreeNode(int val, TreeNode left, TreeNode right) {
* this.val = val;
* this.left = left;
* this.right = right;
* }
* }
*/
/*
“102.二叉树的层序遍历”，奇数层正序插入节点值，偶数层逆序插入节点值
*/
class Solution {
List<List<Integer>> list = new ArrayList<>();
public List<List<Integer>> zigzagLevelOrder(TreeNode root) {
dfs(root, 1);
return list;
}
public void dfs(TreeNode root, int layer) {
if (root == null) {
return;
}
if (list.size() < layer) {
list.add(new ArrayList<>());
}
if (layer % 2 == 1) {
list.get(layer - 1).add(root.val);
} else {
list.get(layer - 1).add(0, root.val);
}
dfs(root.left, layer + 1);
dfs(root.right, layer + 1);
}
}
/*
“102.二叉树的层序遍历”，加上是否反转标记，奇数层不变，偶数层反转子数组
*/
class Solution {
public List<List<Integer>> zigzagLevelOrder(TreeNode root) {
List<List<Integer>> list = new ArrayList<>();
if (root == null) {
return list;
}
Queue<TreeNode> queue = new LinkedList<>();
queue.add(root);
boolean flag = false;
while (!queue.isEmpty()) {
int count = queue.size();
List<Integer> sonList = new ArrayList<>();
while (count > 0) {
TreeNode node = queue.remove();
sonList.add(node.val);
if (node.left != null) {
queue.add(node.left);
}
if (node.right != null) {
queue.add(node.right);
}
count--;
}
if (flag) {
Collections.reverse(sonList);
}
list.add(sonList);
flag = !flag;
}
return list;
}
}