其实很简单，successor上来为null。如果p在root左边，就动successor＝root，右边就不动，直到root为null。
public TreeNode inorderSuccessor(TreeNode root, TreeNode p) {
    TreeNode succ = null;
    while (root != null) {
        if (p.val < root.val) {
            succ = root;
            root = root.left;
        }
        else
            root = root.right;
    }
    return succ;
}
----分割线
public class Solution {
    private TreeNode dummy = new TreeNode(-1);
    public TreeNode inorderSuccessor(TreeNode root, TreeNode p) {
        if (root == null) {
            return root;
        }
        if (p == null) {
            return p;
        }
        // write your code here
        TreeNode firstRightAncestor = dummy;
        TreeNode parent = dummy;
        return successorHelper(root, p, firstRightAncestor, parent);
    }
    private TreeNode successorHelper(TreeNode root, TreeNode p,
        TreeNode firstRightAncestor, TreeNode parent) {
            if (root == null) {
                return null;
            }
            if (root == p) {
                if (p.right != null) {
                    // p has a right subtree
                    return findSmallest(root.right);
                } else if (parent == dummy) {
                    // p doesn't have any ancestor
                    return null;
                } else if (parent.right == p) {
                    return firstRightAncestor == dummy ? null : firstRightAncestor;
                }
                return parent;
            }
            parent = root;
            // update firstRightAncestor
            if (p.val < root.val) {
                firstRightAncestor = root;
                root = root.left;
            } else {
                root = root.right;
            }
            return successorHelper(root, p, firstRightAncestor, parent);
        }
    private TreeNode findSmallest(TreeNode root) {
        return root.left == null ? root : findSmallest(root.left);
    }
}