515. Find Largest Value in Each Tree Row

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

Example:
Input: 

          1
         / \
        3   2
       / \   \  
      5   3   9 

Output: [1, 3, 9]

经典bfs, 不多说

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public List<Integer> largestValues(TreeNode root) {
        
        List<Integer> ans = new ArrayList<>();
        if (root == null) return ans;
        Queue<TreeNode> q = new LinkedList<>();
        
        q.offer(root);
        while (!q.isEmpty()) {
            int size = q.size();
            int max = Integer.MIN_VALUE;
            while (size > 0) {
                 TreeNode cur = q.poll();
                 size--;
                 max = Math.max(cur.val, max);
                 if (cur.left != null) {
                    q.offer(cur.left);
                }
                if (cur.right != null) {
                    q.offer(cur.right);
                }
            }
            ans.add(max);
        }
        return ans;
    }
}