BFS用deque来实现，因为deque是FIFO的性质，所以可以通过deque.front()拿到头上的item，然后pointer指着头item,找到他的左右孩子。

传pointer进deque，可以联系起deque和binary tree的联系。deque<TreeNode*>mydeque。

#include <iostream>
#include <deque>
using namespace std;
struct TreeNode{
    int val;
    TreeNode* left;
    TreeNode* right;
    TreeNode(int x):val(x),left(NULL),right(NULL){}
};
TreeNode* newNode(int n){
    TreeNode* node = new TreeNode(n);
    //node->val = n;
    node->left = NULL;
    node->right = NULL;
    return node;
}
TreeNode* insert(TreeNode* node, int n){
    if (node == NULL){
        return (newNode(n));
    }
    else{
        if(n<=node->val){
            node->left = insert(node->left, n);
        }
        else{
            node->right = insert(node->right,n);
        }
    }
    return node;
}
void BFSaTree(TreeNode* root,deque<TreeNode*>& mydeque)
{
    if(root==NULL){
        return;
    }
    mydeque.push_back(root);
    TreeNode* p_root = root;
    while(!mydeque.empty())
    {
        if(p_root->left!=NULL){
            mydeque.push_back(p_root->left);
        }
        if(p_root->right!=NULL){
            mydeque.push_back(p_root->right);
        }
        cout<<mydeque.front()->val<<endl;
        mydeque.pop_front();
        p_root = mydeque.front();
    }
}
int main()
{
    TreeNode* node;
    //node = insert(node, 2);
    // node = insert(node, 3);
    // node = insert(node, 1);
    // node = insert(node, 1);
    // node = insert(node, 8);
    // node = insert(node, 9);
    deque<TreeNode*>q;
    BFSaTree(node,q);
    return 0;
}

github的地址：https://github.com/will-I-amor/binaryTree/blob/master/BFS_a_Tree.cpp