代码随想录训练营第三十期|第十四天|二叉树part01|● 理论基础● 递归遍历 ● 迭代遍历● 统一迭代

144. 二叉树的前序遍历 - 力扣（LeetCode）

/**
 * 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> res = new ArrayList<>();
        if (root == null) return res;
        traversal(root, res);
        return res;
    }

    private void traversal(TreeNode root, List<Integer> res) {
        if (root == null) return;
        res.add(root.val);
        traversal(root.left, res);
        traversal(root.right, res);
    }
}

145. 二叉树的后序遍历 - 力扣（LeetCode）

/**
 * 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> res = new ArrayList<>();
        if (root == null) return res;
        traversal(root, res);
        return res;
    }

    private void traversal(TreeNode root, List<Integer> res) {
        if (root == null) return;
        traversal(root.left, res);
        traversal(root.right, res);
        res.add(root.val);
    }
}

94. 二叉树的中序遍历 - 力扣（LeetCode）

/**
 * 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> res = new ArrayList<>();
        if (root == null) return res;
        traversal(root, res);
        return res;
    }

    private void traversal(TreeNode root, List<Integer> res) {
        if (root == null) return;
        traversal(root.left, res);
        res.add(root.val);
        traversal(root.right, res);
    }
}