二叉树前中后序遍历（迭代做法）

二叉树前中后序遍历（迭代做法）

二叉树的前中后序遍历用递归来做的话很简单，用迭代来做，其实就是把递归的过程用栈模拟出来。

二叉树的前序遍历

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

二叉树的中序遍历

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

二叉树的后序遍历

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;
    }
};