GuoXin Li's Blog

Iteratively. 144.Binary Tree Preorder&Inorder Traversal

字数统计: 395阅读时长: 2 min
2021/02/01 Share

144. Binary Tree Preorder Traversal

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

Example 1:

Input: root = [1,null,2,3]
Output: [1,2,3]
Example 2:

Input: root = []
Output: []
Example 3:

Input: root = [1]
Output: [1]
Example 4:

Input: root = [1,2]
Output: [1,2]
Example 5:

Input: root = [1,null,2]
Output: [1,2]

Constraints:

The number of nodes in the tree is in the range [0, 100].
-100 <= Node.val <= 100

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

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/**
 * 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) {
        vector<int> res;
        if (root == nullptr) {
            return res;
        }

        stack<TreeNode*> stk;
        TreeNode* node = root;
        while (!stk.empty() || node != nullptr) {
            while (node != nullptr) {
                res.emplace_back(node->val);
                stk.emplace(node);
                node = node->left;
            }
            if(!stk.empty()){
                node = stk.top();
                stk.pop();
                node = node->right;
            }
        }
        return res;
    }
};

Iterative Method

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void PreOrderTraversal(BinTree BT){
    BinTree T = BT;
    Stack S = CreatStack(Maxsize);	//建立一个堆栈 S
    While(T || !IsEmpty(S)){		//当 T 存在或者 S 非空时，S 为空时说明退完了
        while(T){					//当 T 存在时
            Push(S,T);
            printf("%5d", T->Data);
            T = T->Left;
        }
        if(!IsEmpty(S)){			//如果 S 非空 
          	T = S.top();
            S.pop();		
            T = T->Right;
        }
    }
}

difference with Inorder traversal

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void PreOrderTraversal(BinTree BT){
    BinTree T = BT;
    Stack S = CreatStack(Maxsize);	//建立一个堆栈 S
    While(T || !IsEmpty(S)){		//当 T 存在或者 S 非空时，S 为空时说明退完了
        while(T){					//当 T 存在时
            Push(S,T);
            T = T->Left;
        }
        if(!IsEmpty(S)){			//如果 S 非空 
          	T = S.top();
            printf("%5d", T->Data);
            S.pop();		
            T = T->Right;
        }
    }
}
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