leetcode124.二叉树中的最大路径和

给定一个非空二叉树，返回其最大路径和。

本题中，路径被定义为一条从树中任意节点出发，达到任意节点的序列。该路径至少包含一个节点，且不一定经过根节点。

示例1：

输入: [1,2,3]

       1
      / \
     2   3

输出: 6

示例2：

输入: [-10,9,20,null,null,15,7]

   -10
   / \
  9  20
    /  \
   15   7

输出: 42

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public int maxPathSum(TreeNode root) {
        map.put(null, 0);
        getMax(root);
        Queue<TreeNode> queue = new LinkedList<>();
        queue.offer(root);
        int res = Integer.MIN_VALUE;
        while(!queue.isEmpty()) {
            TreeNode poll = queue.poll();
            TreeNode left = poll.left;
            TreeNode right = poll.right;
            if(left != null) queue.offer(left);
            if(right != null) queue.offer(right);
            res = Integer.max(res, (map.get(left) > 0 ? map.get(left) : 0) +(map.get(right) > 0 ? map.get(right) :0) +poll.val);
        }
        return res;
    }

    Map<TreeNode, Integer> map = new HashMap<>();

    public int getMax(TreeNode root) { // 任意节点到叶子节点最大和
        if(map.get(root) != null) return map.get(root);

        if(root.left == null && root.right == null) {
            map.put(root, root.val);
            return root.val;
        }
        
        int leftTree;
        if(map.get(root.left) != null) leftTree = map.get(root.left);
        else {
            leftTree = getMax(root.left);
            map.put(root.left, leftTree);
        }

        int rightTree;
        if(map.get(root.right) != null) rightTree = map.get(root.right);
        else {
            rightTree = getMax(root.right);
            map.put(root.right, rightTree);
        }

        int maxTree = Integer.max(leftTree, rightTree);
        int nowTree = root.val+(maxTree > 0 ? maxTree : 0);
        map.put(root, nowTree);
        return nowTree;
    }
}