[leetcode]543. Diameter of Binary Tree二叉树的直径

int res = 1;
    public int diameterOfBinaryTree(TreeNode root) {
        helper(root);
        return res-1;
    }
    public int[] helper(TreeNode root)
    {
        //两个量分别是节点深度，节点最大diameter
        if (root==null) return new int[]{0,0};
        int cur = 0;
        int[] l = helper(root.left);
        int[] r = helper(root.right);
        cur = Math.max(Math.max(l[1],r[1]),l[0]+r[0]+1);
        res = Math.max(res,cur);
        return new int[]{Math.max(l[0],r[0])+1,cur};
    }