Talk is cheap.

/**
 * Definition for a binary tree node.
 * type TreeNode struct {
 *     Val int
 *     Left *TreeNode
 *     Right *TreeNode
 *     Father *TreeNode
 * }
 */
func inorderSuccessor(p *TreeNode) *TreeNode {
    if p == nil {
        return nil
    }
    if p.Right != nil {
        p = p.Right
        for p.Left != nil {
            p = p.Left
        }
        return p
    }
    for p.Father != nil {
        if p.Father.Left == p {
            return p.Father
        } else {
            p = p.Father
        }
    }
    return nil
}