先序遍历

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

中序遍历

vector<int> inorderTraversal(TreeNode* root) {
        if(!root) return {};
        vector<int> result;
        stack<TreeNode*> stk;
        stk.push(root);
        while(!stk.empty()){
            TreeNode* node = stk.top();
            stk.pop();
            if(node){
                if(node -> right){
                    stk.push(node -> right);
                }
                stk.push(node);
                stk.push(nullptr);
                if(node -> left){
                    stk.push(node -> left);
                } 
            }else{
                result.push_back(stk.top()->val);
                stk.pop();
            }
        }
        return result;
    }

后序遍历

vector<int> postorderTraversal(TreeNode* root) {
        if(!root) return {};
        vector<int> result;
        stack<TreeNode*> stk;
        stk.push(root);
        while(!stk.empty()){
            TreeNode* node = stk.top();
            stk.pop();
            if(node){
                stk.push(node);
                stk.push(nullptr);
                if(node -> right){
                    stk.push(node -> right);
                }
                if(node -> left){
                    stk.push(node -> left);
                } 
            }else{
                result.push_back(stk.top()->val);
                stk.pop();
            }
        }
        return result;
    }