代码如下：

#include <iostream>
#include <vector>

using namespace std;

typedef struct Node
{
    int data;
    Node* pLeft;
    Node* pRight;

    Node(const int& data) {
        this->data = data;
        pLeft = NULL;
        pRight = NULL;
    }
} *PNode;

void printTreeByLevel(PNode node) {
    vector<PNode> vec;
    if (node == NULL) {
        return;
    }
    vec.push_back(node);
    int i = 0;
    int preCount = 1;
    int count = 0;
    int cur = 0;
    while (i < vec.size()) {
        cout << vec[i]->data << "  ";
        
        if (vec[i]->pLeft != NULL)
        { 
            vec.push_back(vec[i]->pLeft);
            count++;
        }
        if (vec[i]->pRight != NULL)
        {
            vec.push_back(vec[i]->pRight);
            count++;
        }  

        i++;
        if (i - cur == preCount)
        {
            cout << endl;
            preCount = count;
            cur = i;
            count = 0;
        }
    }

    cout << endl;
}

void main() {
    PNode node1 = new Node(1);

    PNode node2 = new Node(2);
    PNode node3 = new Node(3);

    node1->pLeft = node2;
    node1->pRight = node3;

    PNode node4 = new Node(4);
    PNode node5 = new Node(5);
    node2->pLeft = node4;
    node2->pRight = node5;

    PNode node6 = new Node(6);
    node3->pRight = node6;

    PNode node7 = new Node(7);
    PNode node8 = new Node(8);
    node5->pLeft = node7;
    node5->pRight = node8;

    PNode node9 = new Node(9);
    PNode node10 = new Node(10);
    node6->pLeft = node9;
    node6->pRight = node10;

    PNode node11 = new Node(11);
    node7->pLeft = node11;

    PNode node12 = new Node(12);
    node8->pRight = node12;

    printTreeByLevel(node1);

    getchar();
}

解法二：

void printTreeLevel(TreeNode* root)
{
    vector<TreeNode*> vec;
    vec.push_back(root);
    int cur = 0;
    int last = 1;
    while (cur < vec.size())
    {
        int count = 0;
        while (cur < last)
        {
            cout << vec[cur]->m_data<<" ";
            if (vec[cur]->pLChild != NULL)
            {
                count++;
                vec.push_back(vec[cur]->pLChild);
            }

            if (vec[cur]->pRChild != NULL)
            {
                count++;
                vec.push_back(vec[cur]->pRChild);
            }
            cur++;
        }
        last += count;
        cout << endl;
    }
}

方法三：

vector<vector<int> > PrintNew(TreeNode* pRoot) {
    vector<vector<int> > vecList;
    queue<TreeNode*> queList;
    queList.push(pRoot);
    while (queList.size() > 0)
    {
        int start = 0;
        int end = queList.size();
        vector<int> vec;
        while (start < end)
        {           
            TreeNode* node = queList.front();
            queList.pop();
            vec.push_back(node->m_data);
            if (node->pLChild != NULL)
            {
                queList.push(node->pLChild);
            }
            if (node->pRChild != NULL)
            {
                queList.push(node->pRChild);
            }
            ++start;
        }
        vecList.push_back(vec);
    }
    
    return vecList;
}

引自《编程之美》