515_在每个树行中找最大值.md

import java.util.*;

/**
 * @author : xfhy
 * Create time : 2020年11月6日08:17:09
 * Description : 515. 在每个树行中找最大值
 * source : https://leetcode-cn.com/problems/find-largest-value-in-each-tree-row/
 */
public class Solution {

    public static class TreeNode {
        int val;
        TreeNode left;
        TreeNode right;

        TreeNode(int val) {
            this.val = val;
        }

        TreeNode(int val, TreeNode left, TreeNode right) {
            this.val = val;
            this.left = left;
            this.right = right;
        }

        @Override
        public String toString() {
            return "TreeNode{" +
                    "val=" + val +
                    ", left=" + left +
                    ", right=" + right +
                    '}';
        }
    }

    /**
     * 思路: 二叉树层序遍历,找到每一层的最大值汇总即可
     */
    public static List<Integer> largestValues(TreeNode root) {
        List<Integer> res = new LinkedList<>();
        if (root == null) {
            return res;
        }

        Queue<TreeNode> queue = new LinkedList<>();
        queue.offer(root);
        while (!queue.isEmpty()) {
            int size = queue.size();
            int max = queue.peek().val;
            for (int i = 0; i < size; i++) {
                TreeNode poll = queue.poll();
                if (poll.val > max) {
                    max = poll.val;
                }
                if (poll.left != null) {
                    queue.offer(poll.left);
                }
                if (poll.right != null) {
                    queue.offer(poll.right);
                }
            }
            res.add(max);
        }
        return res;
    }

    public static TreeNode createBinaryTree(LinkedList<Integer> inputList) {
        if (inputList == null || inputList.isEmpty()) {
            return null;
        }
        TreeNode node = null;
        Integer data = inputList.removeFirst();
        if (data != null) {
            node = new TreeNode(data);
            node.left = createBinaryTree(inputList);
            node.right = createBinaryTree(inputList);
        }
        return node;
    }

    public static void main(String[] args) {
        /*LinkedList<Integer> integers = new LinkedList<>(Arrays.asList(5, 4, 11, 7, null, null, 2, null, null, null, 8, 13, null, null, 4,
                5, null, null, 1));*/
        /*LinkedList<Integer> integers = new LinkedList<>(Arrays.asList(3, 2, 9, null, null, 10, null,
                null, 8, null, 4));*/
        LinkedList<Integer> integers = new LinkedList<>(Arrays.asList(5, 2, null, null, 3));
        TreeNode binaryTree = createBinaryTree(integers);

        List<Integer> res = largestValues(binaryTree);
        System.out.println(res);
    }

}