687. Longest Univalue Path

Given a binary tree, find the length of the longest path where each node in the path has the same value. This path may or may not pass through the root.

Note: The length of path between two nodes is represented by the number of edges between them.

Note: The given binary tree has not more than 10000 nodes. The height of the tree is not more than 1000.

//注意长度指的是两个节点（两个节点可以在一个节点的左右两边）间边的个数，

class Solution {

    public int longestUnivaluePath(TreeNode root) {

        int[] res = new int[1];

        if (root != null)

            dfs(root, res);

        return res[0];

    }

    

     private int dfs(TreeNode node, int[] res) {

        if (node == null)

            return 0;

        int l = dfs(node.left, res);

        int r = dfs(node.right, res);

        int resl, resr;

        if (node.left != null && node.left.val == node.val )

            resl = l + 1;

        else

            resl = 0;

        if (node.right != null && node.right.val == node.val )

            resr = r + 1;

        else

            resr = 0;

        res[0] = Math.max(res[0], resl + resr);

        return Math.max(resl, resr);                      

    }

}