先序遍历

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:
    vector<int> preorderTraversal(TreeNode* root) {
        vector<int> res;
        stack<TreeNode*> s;
        while(root != nullptr || !s.empty()){
            while(root != nullptr){
                s.push(root);
                res.push_back(root->val);
                root = root->left;
            }
            root = s.top();
            s.pop();
            root = root->right;
        }
        return res;
    }
};

中序遍历

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:
    vector<int> inorderTraversal(TreeNode* root) {
        vector<int> res;
        stack<TreeNode* > s;
        while(root != nullptr || !s.empty()){
            while(root != nullptr){ // 若有左节点一直入栈
                s.push(root);
                root = root->left;
            }
            root = s.top();
            s.pop();
            res.push_back(root->val);
            root = root->right; // 访问右结点
        }
        return res;
    }
};

后序遍历

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
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){ // 若有左节点一直入栈
                stk.push(root);
                root = root->left;
            }

            root = stk.top();
            stk.pop();
            if(root->right == nullptr || root->right == prev){ //如果为叶子结点或者
                res.push_back(root->val);
                prev = root;
                root = nullptr; // 去掉回溯 不然会一直循环
            } else {
                // 若有右结点则入栈(一层)
                stk.push(root);
                root = root->right;
            }
        }
        return res;
    }
};