leecode刷题-1

树的前中后序非递归算法

先序遍历-1

class Solution {
public:
    vector<int> preorderTraversal(TreeNode* root) {
        vector<int> result;
        stack <TreeNode*> st;
        TreeNode* node=root;
        while(!st.empty()||node!=nullptr){
            while(node!=nullptr){
                st.push(node);
                result.push_back(node->val);
                node=node->left;
            }
            if(!st.empty()){
                node=st.top();
                st.pop();
                node=node->right;
            }
        }
        return result;

    }
};

先序遍历-2

class Solution {
public:
    vector<int> preorderTraversal(TreeNode* root) {
        vector<int> result;
        stack <TreeNode*> st;
        TreeNode* node=root;
        unordered_set<TreeNode*>  vis;
        if(root==nullptr){
            return result;
        }
        st.push(root);
        while(!st.empty()){
            node=st.top();
            if(vis.count(node)){
                st.pop();
            }
            else{
                vis.insert(node);
                result.push_back(node->val);
                if(node->right){
                    st.push(node->right);
                }
                if(node->left){
                    st.push(node->left);
                }
            }

        }
        return result;

    }
};

##中序遍历-1

class Solution {
public:
    vector<int> inorderTraversal(TreeNode* root) {
        vector<int> res;
        stack<TreeNode*> stk;
        while (root != nullptr || !stk.empty()) {
            while (root != nullptr) {
                stk.push(root);
                root = root->left;
            }
            root = stk.top();
            stk.pop();
            res.push_back(root->val);
            root = root->right;
        }
        return res;
    }
};

后序遍历——1

class Solution {
public:
    vector<int> postorderTraversal(TreeNode *root) {
        vector<int> res;
        if (root == nullptr) {
            return res;
        }

        stack<TreeNode *> stk;
        TreeNode *prev = nullptr;
        while (root != nullptr || !stk.empty()) {
            while (root != nullptr) {
                stk.emplace(root);
                root = root->left;
            }
            root = stk.top();
            stk.pop();
            if (root->right == nullptr || root->right == prev) {
                res.emplace_back(root->val);
                prev = root;
                root = nullptr;
            } else {
                stk.emplace(root);
                root = root->right;
            }
        }
        return res;
    }
};

层次遍历

public:
    vector<vector<int>> levelOrder(TreeNode* root) {
        vector <vector <int>> ret;
        if (!root) {
            return ret;
        }

        queue <TreeNode*> q;
        q.push(root);
        while (!q.empty()) {
            int currentLevelSize = q.size();
            ret.push_back(vector <int> ());
            for (int i = 1; i <= currentLevelSize; ++i) {
                auto node = q.front(); q.pop();
                ret.back().push_back(node->val);
                if (node->left) q.push(node->left);
                if (node->right) q.push(node->right);
            }
        }
        
        return ret;
    }
};