代码随想录算法训练营第十四天 | 二叉树part01

题目：

144.二叉树的前序遍历，145.二叉树的后序遍历，94.二叉树的中序遍历

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学习内容：

写递归的三步骤：

• 确定递归函数（往往要自己写）的参数和返回值；
• 确定递归函数的终止条件；
• 确定单层递归的逻辑；

144.二叉树的前序遍历

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     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;
 *     }
 * }
 */
class Solution {
    public List<Integer> preorderTraversal(TreeNode root) {
        List<Integer> result = new ArrayList<Integer> ();
        preorder(root, result);
        return result;
    }
    public void preorder(TreeNode root, List<Integer> result) { // 1.定参数和返回值
        if (root == null) {
            return; // 2.定终止条件
        }
        // 3.定单层递归逻辑（中左右）
        result.add(root.val);
        preorder(root.left, result);
        preorder(root.right, result);
    }
}

145.二叉树的后序遍历

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     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;
 *     }
 * }
 */
class Solution {
    public List<Integer> postorderTraversal(TreeNode root) {
        List<Integer> result = new ArrayList<Integer>();
        postorder(root, result);
        return result;
    }
    public void postorder(TreeNode root, List<Integer> result) {
        if (root == null) {
            return;
        }
        postorder(root.left, result); // 左
        postorder(root.right, result); // 右
        result.add(root.val); // 中
    }
}

94.二叉树的中序遍历

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     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;
 *     }
 * }
 */
class Solution {
    public List<Integer> inorderTraversal(TreeNode root) {
        List<Integer> result = new ArrayList<Integer> ();
        inorder(root, result);
        return result;
    }
    public void inorder(TreeNode root, List<Integer> result) {
        if (root == null) {
            return;
        }
        inorder(root.left, result); // 左
        result.add(root.val); // 中
        inorder(root.right, result); // 右
    }
}

总结

写递归的三步骤：

• 确定递归函数（往往要自己写）的参数和返回值；
• 确定递归函数的终止条件；
• 确定单层递归的逻辑；

学习时间：

2024.3.19