Tree Amplitude

Given a tree of N nodes, return the amplitude of the tree
就是从 root 到 leaf max - min 的差

class TreeNode {
    int val;
    TreeNode left;
    TreeNode right;
    TreeNode (int x) {
        val = x;
        left = null;
        right = null;
    }
}

public class TreeAmplitude {
    public int Solution(TreeNode root) {
        if (root == null) return 0;
        return helper(root, root.val, root.val);
    }

    private int helper(TreeNode root, int min, int max) {
        if (root == null)  return max - min;

        if (root.val < min) min = root.val;
        if (root.val > max) max = root.val;

        return Math.max(helper(root.left, min, max), helper(root.right, min, max));
    }
}