二叉树的前序、中序、后序、层序遍历（C++）

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */

前序遍历

void PreOrder(TreeNode* currentNode)
{
    if(currentNode)
    {
    	cout << currentNode -> val;
        PostOrder(currentNode->left);
        PostOrder(currentNode->right);
    }
}

中序遍历

void InOrder(TreeNode* currentNode)
{
    if(currentNode)
    {
        PostOrder(currentNode->left);
        cout << currentNode -> val;
        PostOrder(currentNode->right);
    }
}

后序遍历

void PostOrder(TreeNode* currentNode)
{
    if(currentNode)
    {
        PostOrder(currentNode->left);
        PostOrder(currentNode->right);
        cout << currentNode -> val;
    }
}

层序遍历

void LevalOrder(TreeNode* root, queue<TreeNode*> q)//层序遍历  显示一个之前先把其左右孩子放到队列里
{
    TreeNode* currentNode = root;
    cout << currentNode -> val;
    while(currentNode)
    {
        if(currentNode->left){
            q.push(currentNode->left);  //入队列
        }
        if(currentNode->right){
            q.push(currentNode->right); //入队列
        }
        if(q.empty())  return;
        currentNode=q.front();//将队列头记下，将左右孩子入队列
        q.pop();//出队头
    }
}