二叉树 Z 字型遍历

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考察点：遍历

<pre><code class=""language-java"" lang=""java"">public List<List<Integer>> zigzagLevelOrder(TreeNode root) {
List<List<Integer>> res =new ArrayList<List<Integer>>();
LinkedList<TreeNode> l = new LinkedList<TreeNode>();
boolean flag = true;
if (root == null) {
return res;
}
l.add(root);
while (l.size() != 0) {
flag = !flag;
int size = l.size();
List<Integer> list = new ArrayList<Integer>();
for (int i = 0; i < size; i++) {
TreeNode temp = l.remove();
list.add(temp.val);
if (temp.left != null)
l.add(temp.left);
if (temp.right != null)
l.add(temp.right);
}
if (flag) {
Collections.reverse(list);
}
res.add(list);
}
return res;
}
</code></pre>

<pre><code> "“`