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]
题目含义：从上到下找到每行的最大值
方法一：DFS
 参数d表示层数的索引，第一行对应的d为0

    private void largestValue(TreeNode root, List<Integer> res, int d){
        if(root == null){
            return;
        }
        //expand list size
        if(d == res.size()){
            res.add(root.val); //在最下面添加新的行
        }
        else{
            //or set value
            res.set(d, Math.max(res.get(d), root.val)); //当前节点值处于第(d+1)层，如果当前值大于res中保存的值，则替换成最大值
        }
        largestValue(root.left, res, d+1);
        largestValue(root.right, res, d+1);
    }
    
    
    public List<Integer> largestValues(TreeNode root) {
        List<Integer> res = new ArrayList<Integer>();
        largestValue(root, res, 0);
        return res;     
    }

方法二：

    public List<Integer> largestValues(TreeNode root) {
        Queue<TreeNode> queue = new LinkedList<TreeNode>();
        List<Integer> res = new ArrayList<Integer>();
        if (root == null) return res;
        queue.add(root);
        while (!queue.isEmpty()) {
            int largestElement = Integer.MIN_VALUE;
            int queueSize = queue.size();
            for (int i=0;i<queueSize;i++) {
                TreeNode cur = queue.poll();
                largestElement = Math.max(cur.val, largestElement);
                if (cur.left != null) queue.add(cur.left);
                if (cur.right != null) queue.add(cur.right);
            }
            res.add(largestElement);
        }
        return res;
    }