使用層序遍歷去遍歷每一個節點，
當該結點為NULL時，str = str+"#!"
當該結點非NULL時，str = str+該節點的data+"!"


#include <iostream>
#include <vector>
#include<queue>
#include <string>
using namespace std;

typedef struct TreeNode
{
    string data;
    struct TreeNode* lchild;
    struct TreeNode* rchild;
}TreeNode;

string
 
 levelSerialize(TreeNode* T)  //層次序列化
{
    if (!T)
        return "#!";

    string str = T->data + "!";
    queue<TreeNode*> Q;
    Q.push(T);

    TreeNode* cur;  //記錄每次的出隊節點

    while (!Q.empty())
    {
        cur = Q.front();
        Q.pop();

        if (cur->lchild)
        {
            str =
 
 str + cur->lchild->data + "!";
            Q.push(cur->lchild);
        }
        else
            str = str + "#!";

        if (cur->rchild)
        {
            str = str + cur->rchild->data + "!";
            Q.push(cur->rchild);
        }
        else
            str = str +
 
 "#!";
    }
    return str;
}


void levelTraver(TreeNode* T)  //層次遍歷
{
    if (!T)
        return;

    queue<TreeNode*> Q;

    TreeNode* cur = T;

    Q.push(cur);
    while (!Q.empty())
    {
        cout << Q.front()->data << " ";
        cur = Q.front();
        Q.pop();
        if (cur->lchild)
            Q.push(cur->lchild);
        if (cur->rchild)
            Q.push(cur->rchild);
    }
}

int main()
{
    TreeNode* s1 = new TreeNode;
    TreeNode* s2 = new TreeNode;
    TreeNode* s3 = new TreeNode;
    TreeNode* s4 = new TreeNode;
    TreeNode* s5 = new TreeNode;
    TreeNode* s6 = new TreeNode;
    TreeNode* s7 = new TreeNode;

    s1->data = "1";
    s1->lchild = s2;
    s1->rchild = s5;

    s2->data = "2";
    s2->lchild = NULL;
    s2->rchild = s3;

    s3->data = "3";
    s3->lchild = s4;
    s3->rchild = NULL;

    s4->data = "4";
    s4->lchild = NULL;
    s4->rchild = NULL;

    s5->data = "5";
    s5->lchild = s6;
    s5->rchild = NULL;

    s6->data = "6";
    s6->lchild = NULL;
    s6->rchild = s7;

    s7->data = "7";
    s7->lchild = NULL;
    s7->rchild = NULL;



    cout << "層序遍歷" << endl;
    levelTraver(s1);
    cout << endl;

    string s = levelSerialize(s1);
    cout << s;

    return 0;

}