遍历二叉树-前/中/后序-递归/非递归

Posted on Edited on In

递归方法

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void preorder(TreeNode* root, vector<int>&path)
{
    if (root==nullptr)	return;
 	path.push_back(root->val);
    preorder(root->left, path);
    preorder(root->right, path);
}
void inorder(TreeNode* root, vector<int>&path)
{
    if (root==nullptr)	return;
    preorder(root->left, path);
    path.push_back(root->val);
    preorder(root->right, path);
}
void postorder(TreeNode* root, vector<int>&path)
{
    if (root==nullptr)	return;
    preorder(root->left, path);
    preorder(root->right, path);
    path.push_back(root->val);
}

非递归方法

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void preorder(TreeNode* root, vector<int>&path)
{
    stack<pair(TreeNode*, bool)> s;
    // initiate stack
    s.push(make_pair(root, false));
    
    bool visited = false;
    TreeNode* p = root;
    while(!s.empty())
    {
        p = s.top.first;
        visited = s.top.second;
        
        s.pop();
        if (p==nullptr)	continue;
        
        if (visited)
        {
            path.push_back(p->val);
        }
        else
        {
            s.push(make_pair(p->right, false));
            s.push(make_pair(p->left, false));
            s.push(make_pair(p, true)); 
        }
    }
}

void inorder(TreeNode* root, vector<int>&path)
{
    stack<pair(TreeNode*, bool)> s;
    // initiate stack
    s.push(make_pair(root, false));
    
    bool visited = false;
    TreeNode* p = root;
    while(!s.empty())
    {
        p = s.top.first;
        visited = s.top.second;
        
        s.pop();
        if (p==nullptr)	continue;
        
        if (visited)
        {
            path.push_back(p->val);
        }
        else
        {
            s.push(make_pair(p->right, false));
            s.push(make_pair(p, true));
            s.push(make_pair(p->left, false));
        }
    }
}

void postorder(TreeNode* root, vector<int>&path)
{
    stack<pair(TreeNode*, bool)> s;
    // initiate stack
    s.push(make_pair(root, false));
    
    bool visited = false;
    TreeNode* p = root;
    while(!s.empty())
    {
        p = s.top.first;
        visited = s.top.second;
        
        s.pop();
        if (p==nullptr)	continue;
        
        if (visited)
        {
            path.push_back(p->val);
        }
        else
        {
            s.push(make_pair(p, true));
            s.push(make_pair(p->right, false));
            s.push(make_pair(p->left, false));
        }
    }
}

Reference: https://blog.csdn.net/czy47/article/details/81254984