题解 | 二叉树中的最大路径和

全局变量统计最大路径和，递归仅返回当前节点与左子树或右子树的最大路径避免路径分叉

import java.util.*;

/*
 * public class TreeNode {
 *   int val = 0;
 *   TreeNode left = null;
 *   TreeNode right = null;
 *   public TreeNode(int val) {
 *     this.val = val;
 *   }
 * }
 */

public class Solution {
    /**
     * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
     *
     *
     * @param root TreeNode类
     * @return int整型
     */

    int maxPath = Integer.MIN_VALUE;
    public int maxPathSum (TreeNode root) {
        // write code here
        max(root);
        return maxPath;
    }
    public int max(TreeNode root) {
        int leftMaxPathSum = 0, rightMaxPathSum = 0;
        if (root.left != null) leftMaxPathSum = max(root.left);
        if (root.right != null) rightMaxPathSum = max(root.right);
        maxPath = Math.max(maxPath, root.val);
        maxPath = Math.max(maxPath, root.val + leftMaxPathSum + rightMaxPathSum);
        maxPath = Math.max(maxPath, root.val + leftMaxPathSum);
        maxPath = Math.max(maxPath, root.val + rightMaxPathSum);
        int res = Math.max(root.val, root.val + leftMaxPathSum);
        res = Math.max(res, root.val + rightMaxPathSum);
        return res;
    }

}