最简单的思路就是简单的层序遍历 + Collections反转 也可以分成left2righ和right2left两种情况讨论, 让每次切换遍历方向的时候, 也更换push和pop的对应的方向, 还有先left还是先right
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| 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;
}
}
class Solution {
public List<List<Integer>> zigzagLevelOrder(TreeNode root) {
if (root == null) return List.of();
List<List<Integer>> ans = new ArrayList<>();
boolean left2right = true;
Deque<TreeNode> st = new ArrayDeque<>();
st.addLast(root);
while (!st.isEmpty()) {
int size = st.size();
List<Integer> curLevel = new ArrayList<>();
for (int i = 0; i < size; i++) {
TreeNode cur = left2right ? st.removeFirst() : st.removeLast();
curLevel.add(cur.val);
if (left2right) {
if (cur.left != null) st.addLast(cur.left);
if (cur.right != null) st.addLast(cur.right);
} else {
if (cur.right != null) st.addFirst(cur.right);
if (cur.left != null) st.addFirst(cur.left);
}
}
ans.add(curLevel);
left2right = !left2right;
}
return ans;
}
}
|
