图解leetcode —— 124. 二叉树中的最大路径和

class Solution {
    /**
     * 最大路径和
     */
    private int sum = Integer.MIN_VALUE;

    public int maxPathSum(TreeNode root) {
        maxGain(root);
        return sum;
    }

    public int maxGain(TreeNode node) {
        if (node == null) {
            return 0;
        }
        // 小于0的权值舍弃，代表不走该节点
        int leftGain = Math.max(maxGain(node.left), 0);
        int rightGain = Math.max(maxGain(node.right), 0);

        int pathGain = leftGain + rightGain + node.val;
        sum = Math.max(sum, pathGain);
        // 返回最大路径，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。
        return node.val + Math.max(leftGain, rightGain);
    }
}

import sys

class Solution:
    def maxPathSum(self, root: TreeNode) -> int:
        def max_gain(node: TreeNode) -> int:
            nonlocal max_sum
            if not node:
                return 0
            left_gain = max(max_gain(node.left), 0)
            right_gain = max(max_gain(node.right), 0)

            path_gain = left_gain + right_gain + node.val
            max_sum = max(max_sum, path_gain)

            return node.val + max(left_gain, right_gain)

        # 负无穷
        max_sum = float(‘-inf‘)
        max_gain(root)
        return max_sum