leetcode124.二叉树中的最大路径和

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    private int result;

    public int maxPathSum(TreeNode root) {
        result = Integer.MIN_VALUE;
        dfs(root);
        return result;
    }

    private int dfs(TreeNode root) {
        if (root == null) {
            return 0;
        } else {
            int lmax = Math.max(0, dfs(root.left));//左孩子的合法路径最大路径和
            int rmax = Math.max(0, dfs(root.right));//右孩子的合法路径最大路径和
            //更新结果
            result = Math.max(result, root.val + lmax + rmax);
            //向父节点返回合法路径中的较大一组
            return root.val + Math.max(lmax, rmax);
        }
    }
}