LeetCode 144. 二叉树的前序遍历

Posted 2021-10-05 Blocking The Sky

给你二叉树的根节点 root ，返回它节点值的 前序 遍历。

1、递归

class Solution {
public:
    vector<int> result;
    void PreOrder(TreeNode* root){
        if(root){
            result.push_back(root->val);
            PreOrder(root->left);
            PreOrder(root->right);
        }
    }
    vector<int> preorderTraversal(TreeNode* root) {
        PreOrder(root);
        return result;
    }
};

2、栈实现非递归

class Solution {
public:
    vector<int> result;
    vector<int> preorderTraversal(TreeNode* root) {
        stack<TreeNode*> s;
        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;
    }
};