leetcode144--二叉树的前序遍历

1. 题意

求二叉树的前序遍历

2. 题解

2.1 递归

比较简单

/**
 * 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:
    void pre(TreeNode *root, vector<int> &ans) {
        if ( nullptr == root)
            return;

        ans.emplace_back( root->val );
        pre(root->left, ans );
        pre(root->right, ans );
    }

    vector<int> preorderTraversal(TreeNode* root) {
        
        vector<int> ans;
        pre(root, ans);

        return ans;
    }
};

2.2 迭代

不断递归寻找左节点，再中间节点，最后右节点。

/**
 * 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) {
        
        stack<TreeNode *> s;
        vector<int> ans;
        
        while ( root || !s.empty()) {

            if (root) {
                s.push(root);
                ans.emplace_back(root->val);
                root = root->left;
                continue;
            }

            root = s.top();
            s.pop();
            root = root->right;

        }

        return ans;
    }
};