[英]C - recursion not working in a Binary Search Tree
I'm implementing a BST in C.我正在 C 中实施 BST。 If the tree only has 2 leaves and nothing in between, recursion works.
如果树只有 2 片叶子,中间什么都没有,递归就可以了。 But as soon as it goes deeper (eg below 3 levels deep) - the recursion breaks.
但是一旦它变得更深(例如深度低于 3 级) - 递归就会中断。
My code:我的代码:
#include <stdio.h>
#include <stdlib.h>
typedef struct node {
int val;
struct node* left;
struct node* right;
} node_t;
void print_tree(node_t* n, int indent) {
//base case, tree leaves
if (n->left == NULL && n->right == NULL) {
printf("%*c", indent, ' ');
printf("%d\n", n->val);
} else {
//print current level
printf("%*c", indent, ' ');
printf("%d\n", n->val);
//then kids
print_tree(n->left, indent+2);
print_tree(n->right, indent+1);
}
}
int bst() {
node_t* n1;
n1 = (node_t*) malloc(sizeof(node_t));
//level 1 children
n1->val = 5;
n1->left = (node_t*) malloc(sizeof(node_t));
n1->right = (node_t*) malloc(sizeof(node_t));
//level 2 children
n1->left->val = 3;
n1->left->left = (node_t*) malloc(sizeof(node_t));
n1->left->right = NULL;
n1->right->val = 10;
n1->right->left = (node_t*) malloc(sizeof(node_t));
n1->right->right = NULL;
//level 3 children
n1->left->left->val = 1;
n1->left->left->left = NULL;
n1->left->left->right = NULL;
n1->right->left->val = 6;
n1->right->left->left = NULL;
n1->right->left->right = NULL;
print_tree(n1, 0);
return 1;
}
What happens:怎么了:
5
3
1
What I want to happen:我想要发生的事情:
5
3
1
10
6
When I run in debug mode I see that for some reason the base case never triggers and so at some point n
itself becomes NULL
and the if statement breaks.当我在调试模式下运行时,我看到由于某种原因基本情况永远不会触发,因此在某些时候
n
本身变成NULL
并且 if 语句中断。
Why is the base case not triggering?为什么基本情况没有触发?
First point :第一点:
In your function print_tree
, you are dereferencing n
without checking if it is NULL
.在您的 function
print_tree
中,您取消引用n
而不检查它是否是NULL
。 This will cause trouble when there are nodes whose one child is NULL
and the other child is not NULL
.当有节点的一个子节点是
NULL
而另一个子节点不是NULL
时,这将导致麻烦。
You should add你应该添加
if (n == NULL) return;
just after刚过
void print_tree(node_t* n, int indent) {
Second point :第二点:
The line线
print_tree(n->right, indent+1);
is wrong and it will make the number of spaces of 10
and 6
fewer than expected.是错误的,它会使
10
和6
的空格数比预期的少。
It should be它应该是
print_tree(n->right, indent+2);
Your recursion eventually reaches the point where it calls print_tree
on the NULL
pointer n1->left->right
and you trigger a segmentation fault by trying to access n1->left->right->left
in the first if
statement of the function.您的递归最终到达它在
NULL
指针n1->left->right
上调用print_tree
的点,并且您通过尝试在 function 的第一个if
语句中访问n1->left->right->left
来触发分段错误。
This might be more of what you want这可能是您想要的更多
void print_tree(node_t* n, int indent) {
if(n == NULL) {
return;
}
printf("%*c", indent, ' ');
printf("%d\n", n->val);
print_tree(n->left, indent+2);
print_tree(n->right, indent+2);
}
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