[LeetCode]Binary Tree Preorder Traversal-优快云博客

本文链接：https://blog.youkuaiyun.com/sbitswc/article/details/26555215

本文详细介绍了如何使用递归和迭代两种方法实现二叉树的前序遍历，并通过示例代码进行了具体演示。重点比较了这两种方法的实现细节、效率和适用场景。

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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?

recursive

java

ArrayList<Integer> result;
	public ArrayList<Integer> preorderTraversal(TreeNode root) {
        result = new ArrayList<Integer>();
        preT(root);
        return result;
	}
	public void preT(TreeNode root){
		if(root!=null){
			result.add(root.val);
			preT(root.left);
			preT(root.right);
		}
	}

c++

void Preorder(TreeNode *root, vector<int> &result){
    if(root!= NULL){
        result.push_back(root->val);
        Preorder(root->left, result);
        Preorder(root->right, result);
    }

}
vector<int> preorderTraversal(TreeNode *root) {

    vector<int> result;
    Preorder(root, result);
    return result;
}

iterative

java

public List<Integer> preorderTraversal(TreeNode root) {
        List<Integer> result = new ArrayList<>();
        if(root==null) return result;
        Stack<TreeNode> path = new Stack<>();
        path.push(root);
        while(!path.isEmpty()){
        	TreeNode topNode = path.pop();
        	result.add(topNode.val);
        	if(topNode.right!=null)
        		path.push(topNode.right);
        	if(topNode.left!=null)
        		path.push(topNode.left);
        }
        return result;
    }

c++

vector<int> preorderTraversal(TreeNode *root) {
    vector<int> path;
    stack<TreeNode*> stk;
    if(root != NULL){
        stk.push(root);
        while(!stk.empty()){
            root = stk.top();
            stk.pop();
            path.push_back(root->val);
            if(root->right) stk.push(root->right);
            if(root->left) stk.push(root->left);
        }
    }
    return path;
}