LeetCode144. 二叉树的前序遍历

题目

分析

用栈来模拟前序遍历，注意右孩子先进左孩子后进，这样出栈顺序就是先左后右！

代码(递归)

/**
 * 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 dfs(TreeNode *root,vector<int> &res){
        if(root == NULL) return;

        res.push_back(root->val);//对根的处理
        dfs(root->left,res);
        dfs(root->right,res);
    }
    vector<int> preorderTraversal(TreeNode* root) {
        vector<int>res;
        dfs(root,res);
        return res;
    }
};

 代码（非递归）

class Solution {
public:
    vector<int> preorderTraversal(TreeNode* root) {
        stack<TreeNode*>s;
        vector<int>res;
        s.push(root);
        while(!s.empty()){
            auto t = s.top();
            s.pop();
            if(t!=NULL){
                res.push_back(t->val);
            }else continue;
            if(t->right!=NULL) s.push(t->right);
            if(t->left!=NULL) s.push(t->left);
        }
        return res;
    }
};