数据结构面试

深度优先遍历（DFS）和广度优先遍历（BFS）

图文详解两种算法：深度优先遍历（DFS）和广度优先遍历（BFS） - 哔哩哔哩

/**
 * 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;
 *     }
 * }
 * 前序遍历
 */
class Solution {

    private List<Integer> res;

    public List<Integer> preorderTraversal(TreeNode root) {
        res = new ArrayList<>();
        preorder(root);
        return res;
    }

    private void preorder(TreeNode root) {
        if (root != null) {
            res.add(root.val);
            preorder(root.left);
            preorder(root.right);
        }
    }
}


/**
 * 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;
 *     }
 * }
 * 后序遍历
 */
class Solution {

    private List<Integer> res;

    public List<Integer> postorderTraversal(TreeNode root) {
        res = new ArrayList<>();
        postorder(root);
        return res;
    }

    private void postorder(TreeNode root) {
        if (root != null) {
            postorder(root.left);
            postorder(root.right);
            res.add(root.val);
        }
    }
}

/**
 * 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;
 *     }
 * }
 * 中序遍历
 */
class Solution {

    private List<Integer> res;

    public List<Integer> inorderTraversal(TreeNode root) {
        res = new ArrayList<>();
        inorder(root);
        return res;
    }

    private void inorder(TreeNode root) {
        if (root != null) {
            inorder(root.left);
            res.add(root.val);
            inorder(root.right);
        }
    }
}