leetcode 515. Find Largest Value in Each Tree Row

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]

解法

层次遍历，每一层入队列，再根据该层的长度分别出队列，再添加出队列的元素对应的左右孩子。直到队列的长度为0。在每一层的操作中，计算最大的元素添加到list中。

/** * 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> ret = new ArrayList<Integer>();        if (root == null) {            return ret;        }        Queue<TreeNode> queue = new LinkedList<TreeNode>();        queue.offer(root);        while (!queue.isEmpty()) {            int max = Integer.MIN_VALUE;            int size = queue.size();            for (int i = 0; i < size; i++) {                TreeNode head = queue.poll();                if (head.val > max) {                    max = head.val;                }                if (head.left != null) {                    queue.offer(head.left);                }                if (head.right != null) {                    queue.offer(head.right);                }            }            ret.add(max);        }        return ret;    }}