代码随想录二叉树——二叉树的递归遍历

题目

二叉树的定义:

public class {
	int val;
	TreeNode left;
	TreeNode right;
	TreeNode () {};
	TreeNode (int val){
		this.val = val;
	}
	TreeNode(int val,TreeNode left,TreeNode right){
		this.val = val;
		this.left = left;
		this.right = right;
	}
}

递归三要素:

  1. 确定递归函数的参数和返回值
  2. 确定终止条件
  3. 确定单层递归的逻辑

java代码:

//前序遍历
class Solution {
	public List<Integer> preorderTraversal(TreeNode root){
		List<Integer> result = new ArrayList<Integer>();
		preorder(root,result);
		return result;
	}
	void preorder(TreeNode root,List<Integer> result){
		if(root == null){
			return;
		}
		result.add(root.val);//中左右
		preorder(root.left,result);
		preorder(root.right,result);
	}
}

//中序遍历
class Solution {
	public List<Integer> inorderTraversal(TreeNode root){
		List<Integer> res = new ArrayList<>();
		inorder(root,res);
		return res;
	}
	void inorder(TreeNode root,List<Integer> result){
		if(root == null) return;
		inorder(root.left,result);//左中右
		result.add(root.val);
		inorder(root.right,result);
	}
}

//后序遍历
class Solution {
	public List<Integer> postorderTraversal(TreeNode root){
		List<Integer> res = new ArrayList<>();
		postorder(root,res);
		return res;
	}
	void postorder(TreeNode root,List<Integer> result){
		if(root == null) return;
		postorder(root.left,result);//左右中
		postorder(root.right,result);
		result.add(root.val);
	}
}

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