二叉树遍历

void preorder(Node * head){
	if(head!=null)
    {
        cout << head -> val;
		preorder(head->left);
        preorder(head->right);
    }
}

void preorder(Node * head){
	if(!head)
        return ;
    stack<Node *> s;
	s.push(head);
    while(!s.empty()){
        Node * tmp = s.top();
        s.pop();
        cout << tmp.val;
        if(tmp->left)
            s.push(tmp->left);
        if(tmp->right)
            s.push(tmp->right);
    }
}

void inorder(Node * head){
	if(head!=null)
    {
		inorder(head->left);
        cout << head -> val;
        inorder(head->right);
    }
}

void inorder(Node * head){//一直将他的左子树压栈。 一直到左子树最左的节点
	if(!head)
		return ;
    stack<Node*> s;
    s.push(head);
    while(!s.empty() || head!=nullptr){
        if(head->left!=nullptr)
            s.push(head->left);
        else{
            head = s.top();
            s.pop();
            cout << head->val;
            if(head->right!=nullptr)
                s.push(head.right);
        }
    }
}

void laterorder(Node * head){
	if(head!=null)
    {
		laterorder(head->left);
        laterorder(head->right);
        cout << head -> val;
    }
}

void laterorder(Node * head){//维护两个栈，第一个栈遍历顺序 中右左; 第二个 左右中。
	if(!head)
        return ;
    stack<Node * > s1,s2;
    s1.push(head);
    while(!s1.empty()){
        head = s1.top();
        s1.pop();
        s2.push(head);
        if(head->left)
            s1.push(head->left);
        if(head->right)
            s1.push(head->right);
    }
    
    while(!s2.empty()){
        head = s2.top();
        s2.pop();
        cout << head->val;
    }
}