二叉树的递归和非递归遍历（前序中序后序层序）

Posted 2021-07-18 Moua

二叉树的前序遍历

递归

class Solution {
public:
    void _preorderTraversal(vector<int>& v,TreeNode* root)
    {
        //递归实现
        if(root == nullptr)
            return;
        v.push_back(root->val);
        _preorderTraversal(v,root->left);
        _preorderTraversal(v,root->right);
    }
    vector<int> preorderTraversal(TreeNode* root) {
        //递归实现
        vector<int> ret;
        _preorderTraversal(ret,root);
        return ret;

    }
};

非递归

class Solution {
public:
    vector<int> preorderTraversal(TreeNode* root) {

        //非递归实现
        vector<int> ret;
        stack<TreeNode*> st;
        TreeNode* cur = root;
        while(cur || !st.empty())
        {
            while(cur)
            {
                st.push(cur);
                ret.push_back(cur->val);
                cur = cur->left;
            }
            //根节点出栈，右子树入栈
            TreeNode* tmp = st.top();
            st.pop();
            cur = tmp->right;
        }
        return ret;
    }
};

二叉树的中序遍历

递归

class Solution {
public:
    //递归
    void _inorderTraversal(vector<int>& v,TreeNode* root)
    {
        if(root == nullptr)
            return;
        _inorderTraversal(v,root->left);
        v.push_back(root->val);
        _inorderTraversal(v,root->right);
    }
    vector<int> inorderTraversal(TreeNode* root) {
        //递归
        vector<int> ret;
        _inorderTraversal(ret,root);
        return ret;
    }
};

非递归

class Solution {
public:
    vector<int> inorderTraversal(TreeNode* root) {
        //非递归
        stack<TreeNode*> st;
        vector<int> ret;
        TreeNode* cur = root;
        while(cur || !st.empty())
        {
            //左子树入栈
            while(cur)
            {
                st.push(cur);
                cur = cur->left;
            }
            //打印左子树和跟节点，并将右子树入栈
            while(!st.empty() && cur == nullptr)
            {
                TreeNode* tmp = st.top();
                st.pop();
                ret.push_back(tmp->val);
                cur = tmp->right;
            }
        }
        return ret;
    }
};

二叉树的后序遍历

递归

class Solution {
public:
    void _postorderTraversal(vector<int>& v,TreeNode* root)
    {
        if(root == nullptr)
            return;
        _postorderTraversal(v,root->left);
        _postorderTraversal(v,root->right);
        v.push_back(root->val);
    }
    vector<int> postorderTraversal(TreeNode* root) {
        //递归
        vector<int> ret;
        _postorderTraversal(ret,root);
        return ret;
    }
};

非递归

class Solution {
public:
    vector<int> postorderTraversal(TreeNode* root) {
        //非递归
        stack<TreeNode*> st;
        vector<int> ret;
        TreeNode* cur = root;
        TreeNode* prev = nullptr;
        while(cur || !st.empty())
        {
            //左子树入栈
            while(cur)
            {
                st.push(cur);
                cur = cur->left;
            }
            //右子树为空或者右子树已经如果栈就打印并出栈
            TreeNode* tmp = st.top();
            if(tmp->right == nullptr || tmp->right == prev)
            {
                ret.push_back(tmp->val);
                st.pop();
                
                prev = tmp;

                cur = nullptr;
            }
            else
            {
                cur = tmp->right;
            }
        }
        return ret;
    }
};

二叉树的层序遍历

二维输出

//方法1
class Solution {
public:
    vector<vector<int>> levelOrder(TreeNode* root) {
        vector<vector<int>> ret;
        if(root == nullptr)
            return ret;
        queue<TreeNode*> parent;
        queue<TreeNode*> child;
        parent.push(root);
        while(!(parent.empty() && child.empty())) 
        {
            //保存parent队里，并将其孩子节点保存到child队列中
            vector<int> v;
            while(!parent.empty())
            {
                TreeNode* tmp = parent.front();
                v.push_back(tmp->val);
                parent.pop();
                if(tmp->left)
                    child.push(tmp->left);
                if(tmp->right)
                    child.push(tmp->right);
            }
            ret.push_back(v);
            //parent = child
            while(!child.empty())
            {
                TreeNode* tmp1 = child.front();
                parent.push(tmp1);
                child.pop();
            }
        }
        return ret;     
    }
};

//方法2
class Solution {
public:
    vector<vector<int>> levelOrder(TreeNode* root) {
        queue<TreeNode*> q;
        vector<vector<int>> ret;
        if(root == nullptr)
            return ret;
        q.push(root);
        while(!q.empty())
        {
            //记录当前层的个数
            int lenght = q.size();
            vector<int> tmp;
            //以个数出当前层，并将孩子节点入队
            while(lenght--)
            {
                TreeNode* head = q.front();
                tmp.push_back(head->val);
                q.pop();
                if(head->left)
                    q.push(head->left);
                if(head->right)
                    q.push(head->right);
            }
            ret.push_back(tmp);
        }
        return ret;
    }
};

从最后一层往前输出

//方法1
class Solution {
public:
    vector<vector<int>> levelOrderBottom(TreeNode* root) {
        queue<TreeNode*> q;
        vector<vector<int>> ret;
        if(root == nullptr)
            return ret;
        q.push(root);
        while(!q.empty())
        {
            //记录当前层的个数
            int lenght = q.size();
            vector<int> tmp;
            //以个数出当前层，并将孩子节点入队
            while(lenght--)
            {
                TreeNode* head = q.front();
                tmp.push_back(head->val);
                q.pop();
                if(head->left)
                    q.push(head->left);
                if(head->right)
                    q.push(head->right);
            }
            ret.insert(ret.begin(),tmp);
        }
        return ret;
    }
};