leetcode题解-144-二叉树的前序遍历

发表于 | 分类于

题目

My way

递归实现

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
private:
    vector<int> vi;
    void helper(TreeNode* root)
    {
        if (!root)  return ;
        vi.push_back(root->val);
        helper(root->left); 
        helper(root->right);
    }
public:
    vector<int> preorderTraversal(TreeNode* root) {
        helper(root);

        return vi;
    }
};

手动压栈实现

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
private:
    vector<int> vi;

public:
    vector<int> preorderTraversal(TreeNode* root) 
    {
        if (root) 
        {
            stack<TreeNode*> s;
            s.push(root);

            while (!s.empty())
            {
                auto top = s.top(); s.pop();
                vi.push_back(top->val);

                if (top->right)    s.push(top->right);
                if (top->left)    s.push(top->left); 
            }
        }   

        return vi;
    }
};

总结

  • 因为前序遍历是 ”根左右“，所以手动入栈的时候是 先右孩子，再左孩子