[数据结构与算法]二叉树先序遍历

非递归堆栈实现

class Solution {
public:
    vector<int> result;
    vector<int> preorderTraversal(TreeNode* root) {
        stack<TreeNode*> s;
        //根左右先序遍历
        while(root||!s.empty()){
            if(root){
                result.push_back(root->val);
                s.push(root);
                root=root->left;
            }
            else{
                root=s.top();
                s.pop();
                root=root->right;
            }
        }
        return result;
    }
};

递归

class Solution {
public:
    vector<int> result;
    vector<int> preorderTraversal(TreeNode* root) {
        //根左右先序遍历
        if(root){
            result.push_back(root->val);
            preorderTraversal(root->left);
            preorderTraversal(root->right);
        }
        return result;
    }
};