leetcode 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]

这题其实就是个广度优先搜索，一般解法是用队列

/**
 * 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) {

        Queue<TreeNode> queue = new LinkedList<>();
        List<Integer> res = new ArrayList<>();
        if(root==null) return res;
        queue.add(root);
        queue.add(null);
        while(!queue.isEmpty()){
            TreeNode p = queue.poll();
            int max = p.val;
            while(p!=null){
                if(p.val > max) max = p.val;
                if(p.left!=null) queue.add(p.left);
                if(p.right!=null) queue.add(p.right);
                p = queue.poll();
            }
            res.add(max);
            if(!queue.isEmpty()) queue.add(null);
        }
        return res;

    }
}

然而因为是每层返回一个值，可以像上一篇一样，记录下到每个节点的深度，只要在改深度上更新即可

public class Solution {
    public List largestValues(TreeNode root) {
        List res = new ArrayList();
        helper(root, res, 0);
        return res;
    }
    private void helper(TreeNode root, List res, int d){
        if(root == null){
            return;
        }
       //expand list size
        if(d == res.size()){
            res.add(root.val);
        }
        else{
        //or set value
            res.set(d, Math.max(res.get(d), root.val));
        }
        helper(root.left, res, d+1);
        helper(root.right, res, d+1);
    }
}