牛客网：NC161 二叉树的中序遍历

dfs

import java.util.*;

/*
 * public class TreeNode {
 *   int val = 0;
 *   TreeNode left = null;
 *   TreeNode right = null;
 *   public TreeNode(int val) {
 *     this.val = val;
 *   }
 * }
 */

public class Solution {
    /**
     * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
     *
     * 
     * @param root TreeNode类 
     * @return int整型一维数组
     */
     List<Integer> res = new ArrayList<>();
    public int[] inorderTraversal (TreeNode root) {
        dfs(root);
        int[] arr = new int[res.size()];
        for (int i = 0; i < arr.length; i++){
            arr[i] = res.get(i);
        }
        return arr;
    }

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

bfs

import java.util.*;

/*
 * public class TreeNode {
 *   int val = 0;
 *   TreeNode left = null;
 *   TreeNode right = null;
 *   public TreeNode(int val) {
 *     this.val = val;
 *   }
 * }
 */

public class Solution {
    /**
     * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
     *
     * 
     * @param root TreeNode类 
     * @return int整型一维数组
     */
    public int[] inorderTraversal (TreeNode root) {
        // write code here
        List<Integer> list = new ArrayList<>();
        Deque<TreeNode> deque = new ArrayDeque<>();
        while (!deque.isEmpty() || root != null){
            while (root != null){
                deque.push(root);
                root = root.left;
            }
            TreeNode node = deque.poll();
            list.add(node.val);
            root = node.right;
        }
        int[] res = new int[list.size()];
        for (int i = 0; i < res.length; i++){
            res[i] = list.get(i);
        }
        return res;
    }
}