编程之递归

struct TreeNode;
typedef struct TreeNode *Postion;
typedef struct TreeNode *SearchTree;

/// 经典之作

SearchTree MakeEmpty(SearchTree T);
Postion Find(ElementType X, SearchTree T);
Postion FindMin(SearchTree T);
Postion FindMax(SearchTree T);
SearchTree Insert(ElementType X, SearchTree T);
SearchTree Delete(ElementType X, SearchTree T);
ElementType Retrieve(Postion P);

struct TreeNode
{
    ElementType Element;
    SearchTree Left;
    SearchTree Right;
};

SearchTree MakeEmpty(SearchTree T) {
    if (T != NULL) {

        MakeEmpty(T->Left);
        MakeEmpty(T->Right);
        free(T);
    }
    return NULL;
}

Postion Find(ElementType X, SearchTree T){
    if (T == NULL) return NULL;
    if (X < T->Element) return Find(X, T-> Left);
    else if (X > T->Element) return Find(X, T->Right);
    else return T;
}

Postion FindMin(SearchTree T){
    if (T == NULL) return NULL;
    else if (T->Left == NULL) return T;
    else return FindMin(T->Left);
}

Postion FindMax(SearchTree T){
    // if (T == NULL) return NULL;
    // else if (T->Right == NULL) return T;
    // else return FindMax(T->Right);
    if (T!= NULL){ // 非递归
        while(T->Right != NULL) {
            T = T->Right;
        }
    }
    return T;
}

SearchTree Insert(ElementType X, SearchTree T){
    if(T == NULL){
        T = malloc(sizeof(struct TreeNode));
        if (T == NULL) FatalError("Out Of Space");
        else {
            T->Element = X;
            T->Left = T->Right = NULL;
        }
    } else if (X < T->Element) {
        T->Left = Insert(X, T->Left);

    } else if (X > T->Element) {
        T->Right = Insert(X, T->Right);
    }
    return T;
}


SearchTree Delete(ElementType X, SearchTree T){ // 此树删除效率不高
    Postion TmpCell;

    if (T == NULL) {
        Error("Element Not Found")
    } else if (X < T->Element){
        T->Left = Delete(X, T->Left);
    } else if(X > T->Element){
        T->Right = Delete(X, T->Right);
    } else if(T->Left && T->Right){ // Two Chil
        TmpCell = FindMin(T->Right);
        T->Element = TmpCell->Element;
        T->Right = Delete(T->Element, T->Right)
    } else { // one or zero chil
        TmpCell = T;
        if (T->Left == NULL) {
            T = T->Right;
        } else if(T->Right == NULL){
            T = T->Left;
        }
        free(TmpCell);
    }
}

这段二叉查找树的代码，我个人特别喜欢，不得不说递归在二叉树方面应用的  就是那么优美，简单，虽然效率上比非递归的低，但是易读性不得不说，nice.

个人觉得这段代码有这个几个地方值得我们学习：

1. 严紧， 就但看if(T == NULL)就可以看出来

2. 单一， 每个函数所处理的东西直接了当，目的清晰明确

3. 简单而又不简单(递归)，聪明的人比较擅长，表示个人不擅长