BFS
/**
* 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;
* }
* }
*/
class Solution {
public List<Integer> largestValues(TreeNode root) {
List<Integer> ansList = new ArrayList<>();
if (root == null) {
return ansList;
}
Queue<TreeNode> queue = new LinkedList<>();
queue.offer(root);
while (!queue.isEmpty()) {
int size = queue.size();
int max = Integer.MIN_VALUE;
for (int i = 0; i < size; i++) {
TreeNode node = queue.poll();
max = Math.max(max, node.val);
if (node.left != null) {
queue.offer(node.left);
}
if (node.right != null) {
queue.offer(node.right);
}
}
ansList.add(max);
}
return ansList;
}
}复杂度分析：
- 时间复杂度：O(n)，每个元素进队出队各一次
- 空间复杂度：O(n)