【算法题】分层遍历二叉树

• 使用双队列方式：一个队列表示当前层，一个队列表示下一层，
  换层时交换指向两队列的指针

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#include <iostream>#include <numeric>#include<algorithm>#include <queue>using namespace std;struct Tree{    double value;    Tree* left;    Tree* right;};void Print(Tree* root){    if (root == NULL)        return;    queue<Tree*> queue_current;    queue<Tree*> queue_next;    auto * p_current = &queue_current;    auto * p_next = &queue_next;    Tree* p = root;    p_next->push(p);    while (!(*p_next).empty())    {        std::swap(p_current, p_next);        while (!(*p_current).empty())        {            p = p_current->front();            p_current->pop();            cout << p->value << " ";            if (p->left != NULL)            {                p_next->push(p->left);            }            if (p->right != NULL)            {                p_next->push(p->right);            }        }        cout << endl;    }}int main(){    Tree root;    Tree node1_1;    Tree node1_2;    Tree node2_1;    Tree node2_2;    Tree node2_3;    Tree node3_1;    Tree node3_2;    Tree node3_3;    Tree node3_4;    root.value = 0;    node1_1.value = 1.1;    node1_2.value = 1.2;    node2_1.value = 2.1;    node2_2.value = 2.2;    node2_3.value = 2.3;    node3_1.value = 3.1;    node3_2.value = 3.2;    node3_3.value = 3.3;    node3_4.value = 3.4;    root.left = &node1_1;    root.right = &node1_2;    node1_1.left = &node2_1;    node1_1.right = &node2_2;    node1_2.left = &node2_3;    node1_2.right = NULL;    node2_1.left = &node3_1;    node2_1.right = &node3_2;    node2_2.left = NULL;    node2_2.right = NULL;    node2_3.left = &node3_3;    node2_3.right = &node3_4;    node3_1.left = NULL;    node3_1.right = NULL;    node3_2.left = NULL;    node3_2.right = NULL;    node3_3.left = NULL;    node3_3.right = NULL;    node3_4.left = NULL;    node3_4.right = NULL;    Print(&root);    return 0;}

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• 使用单队列
  每一层，使用一个变量记录该层的结点个数，也就是队列的当前长度，然后依次在队列中访问该层的结点（将队列中len个元素出队列），并将下一层加入队列。

void Print(Tree* pRoot){    queue<Tree*> q;    if (!pRoot) return;    q.push(pRoot);    while (!q.empty())    {        int len = q.size();        while (len--)        {            Tree* tem = q.front();            q.pop();            cout << tem->value << " ";            if (tem->left) q.push(tem->left);            if (tem->right) q.push(tem->right);        }        cout << endl;    }}