很长时间没有看过数据结构了，二叉树都淡忘了，赶紧复习一下。
二叉树是每个结点最多有两个子树的树结构。下面就定义一下如下：
1 struct tree 2 { 3 int data; 4 struct tree *child_l; 5 struct tree *child_r; 6 };
关于二叉树，有多种遍历方式，前序，中序和后序，前中后代表了根节点的顺序，就是先根，中根，后根。
遍历分为递归遍历和非递归遍历。
递归遍历的代码非常简单。一下就是以前序为例子，进行前序遍历：
1 void read_tree(struct tree *t) { 2 printf("%d ",t->data); 3 if (t->child_l != NULL) 4 read_tree(t->child_l); 5 if (t->child_r != NULL) 6 read_tree(t->child_r); 7 }
非递归需要借助栈的数据结构，那么首先建立一个栈：
1 struct stack { 2 struct tree* stack_value[10]; 3 int top; 4 };
并且初始化一个栈：
1 #define NOMAL_COUNT 1 2 3 struct stack* init_stack() { 4 struct stack *s = (struct stack *) calloc(NOMAL_COUNT, sizeof(struct stack)); 5 s->top = -1; 6 return s; 7 }
然后是入栈和出栈：
1 void stack_push(struct stack *s, struct tree *t) { 2 s->top++; 3 s->stack_value[s->top] = t; 4 } 5 6 struct tree* stack_pop(struct stack *s) { 7 struct tree *t = s->stack_value[s->top]; 8 s->top--; 9 return t; 10 }
接下来就是非递归遍历，非递归就是根完了一直向左，左边完全完了再访问右边：
1 void read_tree2(struct tree *t) { 2 struct tree * temp = t; 3 struct stack *s = init_stack(); 4 5 while (temp != NULL || s->top != -1) { 6 while (temp != NULL) { 7 printf("%d ", temp->data); 8 stack_push(s, temp); 9 temp = temp->child_l; 10 } 11 if (s->top != -1) { 12 temp = stack_pop(s); 13 temp = temp->child_r; 14 } 15 } 16 }
初始化一下这个树：
1 struct tree *t = (struct tree *) calloc(NOMAL_COUNT, sizeof(struct tree)); 2 struct tree *temp; 3 t->data = 1; 4 t->child_l = (struct tree *) calloc(NOMAL_COUNT, sizeof(struct tree)); 5 temp = t->child_l; 6 temp->data = 2; 7 temp->child_l = (struct tree *) calloc(NOMAL_COUNT, sizeof(struct tree)); 8 temp->child_l->data = 4; 9 temp->child_r = (struct tree *) calloc(NOMAL_COUNT, sizeof(struct tree)); 10 temp->child_r->data = 5; 11 t->child_r = (struct tree *) calloc(NOMAL_COUNT, sizeof(struct tree)); 12 t->child_r->data = 3;
然后开始遍历就好啦，得到：1，2，4，5，3.
接下来是二叉树排序：
可以将小的放在左孩子上，大的放在右孩子上：
1 void add_tree(struct tree *t, int number) { 2 if (t->data > number) { 3 if (t->child_l != NULL) { 4 add_tree(t->child_l, number); 5 } else { 6 t->child_l = (struct tree *) calloc(NOMAL_COUNT, sizeof(struct tree)); 7 t->child_l->data = number; 8 } 9 } else { 10 if (t->child_r != NULL) { 11 add_tree(t->child_r, number); 12 } else { 13 t->child_r = (struct tree *) calloc(NOMAL_COUNT, sizeof(struct tree)); 14 t->child_r->data = number; 15 } 16 } 17 }
然后就可以中序遍历二叉树，输出排序结果了：
1 // 中序遍历二叉树 2 void read_tree(struct tree *t) { 3 if (t->child_l != NULL) 4 read_tree(t->child_l); 5 printf("%d ",t->data); 6 if (t->child_r != NULL) 7 read_tree(t->child_r); 8 }
我们来测试一下：
1 int main(int argc, char const *argv[]) 2 { 3 int numbers[] = {8, 7, 1, 4, 3, 5, 9, 19, 13, 11, 15, 20, 16, 2, 6}; 4 struct tree *t = (struct tree *) calloc(NOMAL_COUNT, sizeof(struct tree)); 5 t->data = numbers[0]; 6 for (int i = 1; i < 15; i++) { 7 add_tree(t, numbers[i]); 8 } 9 read_tree(t); 10 return 0; 11 }
输出结果：
1
2
3
4
5
6
7
8
9
11
13
15
16
19
20