二叉树的遍历（4）：层次遍历

#include <iostream>
#include <stack>
#include <queue>
using namespace std;

typedef int TreeNodeElem;

struct TreeNode 
{
    TreeNode* left;
    TreeNode* right;
    TreeNodeElem elem;
};

//层次遍历
void levelOrder(TreeNode* root, int(*visit)(TreeNode*))
{
    TreeNode* p = root; 
    queue<TreeNode*> q;
    if (p != nullptr)
    {
        q.push(p);
    }
    while (!q.empty())
    {
        p = q.front();
        q.pop();
        visit(p);
        if (p->left != nullptr)
        {
            q.push(p->left);
        }
        if (p->right != nullptr)
        {
            q.push(p->right);
        }
    }
}