二叉树递归搜索的return为什么可有可无？

Question

BiTree ifNine(BiTree T){
    if(T != NULL){
        if(T->data == 9){
            return T;
        } else if(T->data < 9){
            return ifNine(T->rChild);
        } else if(T->data > 9){
            return ifNine(T->lChild);
        }
    }
}

 上面这段代码无论加不加return结果都是正确的，这是为什么呢？

以下是完整代码

#include <stdio.h>
#include <malloc.h>
#include <assert.h>
#include <string.h>

typedef struct BiTNode {
    int data;
    struct BTnode *lChild;
    struct BTnode *rChild;
} BiTNode, *BiTree;

BiTree CreatBiTree() {
    BiTree T;
    int num;
    scanf("%d", &num);
    if (num == -1) {
        T = NULL;
    } else {
        T = (BiTNode *) malloc(sizeof(BiTNode));
        T->data = num;
        T->lChild = CreatBiTree();
        T->rChild = CreatBiTree();
    }
    return T;
}

BiTNode *InsertL(BiTNode *parent, int num) {
    BiTNode *ret;
    if (parent->lChild == NULL) {
        ret = parent->lChild = (BiTNode *) malloc(sizeof(BiTNode));
        ret->data = num;
        ret->lChild = NULL;
        ret->rChild = NULL;
        parent->lChild = ret;
        return ret;
    } else {
        BiTNode *tempP = parent->lChild;
        ret = (BiTNode *) malloc(sizeof(BiTNode));
        ret->data = num;
        ret->lChild = tempP;
        ret->rChild = NULL;
        parent->lChild = ret;
        return ret;
    }
}

BiTNode *InsertR(BiTNode *parent, int num) {
    BiTNode *ret;
    if (parent->rChild == NULL) {
        ret = parent->rChild = (BiTNode *) malloc(sizeof(BiTNode));
        ret->data = num;
        ret->lChild = NULL;
        ret->rChild = NULL;
        parent->rChild = ret;
        return ret;
    } else {
        BiTNode *tempP = parent->rChild;
        ret = (BiTNode *) malloc(sizeof(BiTNode));
        ret->data = num;
        ret->rChild = tempP;
        ret->lChild = NULL;
        parent->rChild = ret;
        return ret;
    }
}

void deleteAll(BiTNode *node) {
    if (node != NULL) {
        deleteAll(node->rChild);
        deleteAll(node->lChild);
        free(node);
    }
}

void DeleteL(BiTNode *node) {
    if (node->lChild != NULL) {
        BiTNode *tempP = node->lChild;
        deleteAll(tempP);
        node->lChild = NULL;
    }
}

void DeleteR(BiTNode *node) {
    if (node->rChild != NULL) {
        BiTNode *tempP = node->rChild;
        deleteAll(tempP);
        node->rChild = NULL;
    }
}

BiTree Search(BiTree T, int num) {

}

BiTree ifThree(BiTree T){
    if(T != NULL){
        if(T->data == 9){
            return T;
        } else if(T->data < 9){
            return ifThree(T->rChild);
        } else if(T->data > 9){
            return ifThree(T->lChild);
        }
    }
}

void TraverseQ(BiTree T) {
    if (T != NULL) {
        printf("%d", T->data);
        TraverseQ(T->lChild);
        TraverseQ(T->rChild);
    }
}

void TraverseZ(BiTree T) {
    if (T != NULL) {
        TraverseZ(T->lChild);
        printf("%d", T->data);
        TraverseZ(T->rChild);
    }
}

int main() {
    BiTree T = CreatBiTree();
    TraverseQ(T);
    printf("\n");
    TraverseZ(T);
    printf("\n");


    BiTNode *temp = ifThree(T);
    printf("%d",temp->data);

    return 0;
}

Answer 1

“给定一个小点的输入，完整单步跟踪(同时按Alt+7键查看Call Stack里面从上到下列出的对应从里层到外层的函数调用历史)一遍。”是理解递归函数工作原理的不二法门！
递归函数关注以下几个因素
·退出条件
·参数有哪些
·返回值是什么
·局部变量有哪些
·全局变量有哪些
·何时输出
·会不会导致堆栈溢出