leetcode 515. Find Largest Value in Each Tree Row（二叉树每层的最大值）

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]

给出一棵二叉树，让找出每层的最大值

思路：
按层遍历BFS
需要记住每层有多少个节点，方便记录每层的最大值
同时每层要记录最大值

    public List<Integer> largestValues(TreeNode root) {
        List<Integer> result = new ArrayList<>();
        if(root == null) {
            return result;
        }
        
        int rowLen = 1;
        Queue<TreeNode> queue = new LinkedList<>();
        queue.offer(root);
        
        while(!queue.isEmpty()) {
            int tmpLen = 0;
            int localMax = Integer.MIN_VALUE;
            for(int i = 0; i < rowLen; i++){
                TreeNode tmp = queue.poll();
                localMax = Math.max(localMax, tmp.val);
                if(tmp.left != null) {
                    tmpLen ++;
                    queue.offer(tmp.left);
                }
                
                if(tmp.right != null) {
                    tmpLen ++;
                    queue.offer(tmp.right);
                }
            }
            rowLen = tmpLen;
            result.add(localMax);
        }
        
        return result;
    }