Find Largest Value in Each Tree Row

https://leetcode.com/problems/find-largest-value-in-each-tree-row/description/

You need to find the largest value in each row of a binary tree.

Thoughts

每层最大值自然按层遍历即可。

Code

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public List<Integer> largestValues(TreeNode root) {
        List<Integer> res = new ArrayList<>();
        if (root == null) {
            return res;
        }
        Queue<TreeNode> queue = new LinkedList<>();
        queue.add(root);
        while (!queue.isEmpty()) {
            int length = queue.size();
            int max = Integer.MIN_VALUE;
            for (int i = 0; i < length; i++) {
                TreeNode node = queue.remove();
                max = Math.max(max, node.val);
                if (node.left != null) queue.add(node.left);
                if (node.right != null) queue.add(node.right);               
            }
            res.add(max);
        }

        return res;
    }
}

Analysis

时间复杂度O(n)