LEETCODE: Binary Tree Preorder Traversal

最新推荐文章于 2025-11-25 00:06:26 发布

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#LEETCODE #算法 #遍历

本文介绍了一种不使用递归的二叉树前序遍历算法实现，通过栈来辅助完成节点值的收集。具体步骤包括：首先将根节点的值加入结果向量，并将根节点压入栈中；随后不断处理当前节点的左子树直至为空，过程中继续收集节点值并压栈；最后再处理栈内元素及其右子树。

Given a binary tree, return the preorder traversal of its nodes' values.

For example:
Given binary tree {1,#,2,3},

return [1,2,3].

Note: Recursive solution is trivial, could you do it iteratively?

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    vector<int> results;
    vector<int> preorderTraversal(TreeNode *root) {
        if(root == NULL)    return results;
        stack<TreeNode *> st;
        while(root != NULL) {
            results.push_back(root->val); 
            st.push(root);
            root = root->left;
        }
        while(!st.empty()) {
            TreeNode *top = st.top();
            st.pop();
            if(top->right != NULL) {
                TreeNode* temp = top->right;
                results.push_back(temp->val); 
                st.push(temp);
                temp = temp->left;
                while(temp != NULL) {
                    results.push_back(temp->val);
                    st.push(temp);
                    temp = temp->left;
                }
            }
            
        }
        return results;
    }
};