leetcode-543-Diameter of Binary Tree

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int dfs(root): the max. length of one of substree.

i.e. 
		dfs(root):
			if(root == null)
				return 0
			left += dfs(root->left)
			right += dfs(root->right)
			res = max(res, left + right + 1);
			return max(left, right) + 1;

Error:

1. calculate the path not the node, so we need to -1 at the end.