所谓二叉树的双序遍历就是,对于二叉树的每个节点来说，先访问该节点，再按照这种方式访问左子树，再访问该节点，再按照双序遍历访问右子树。
该方法其实是对前序遍历进行了改变，多增加了一次访问节点函数的调用。
代码如下：

#include <iostream>using namespace std;typedef struct BTNode {char data;BTNode *l_child;BTNode *r_child;
} BTNode;void Double_order(BTNode *t); //双序遍历void Vision(BTNode *t);BTNode *CreateBTree(BTNode *t);void PreOrder(BTNode *t); //前序遍历int main() {auto t = new BTNode;t = CreateBTree(t);PreOrder(t);
//    Double_order(t);return 0;
}BTNode *CreateBTree(BTNode *t) {char data;cin >> data;if (data == '#')t = nullptr; //创建空树else {t = new BTNode;t->data = data;t->l_child = CreateBTree(t->l_child);t->r_child = CreateBTree(t->r_child);}return t;
}void Double_order(BTNode *t) {if (t != nullptr) {Vision(t);Double_order(t->l_child);Vision(t);Double_order(t->r_child);}}void Vision(BTNode *t) {cout << t->data << endl;
}void PreOrder(BTNode *t) {if (t != nullptr) {Vision(t);PreOrder(t->l_child);PreOrder(t->r_child);}}