二叉树的中序遍历

/**
 * 递归实现
 *
 * @param root
 * @return
 */
public static List<Integer> midOrder(TreeNode root) {
    List<Integer> res = new ArrayList<>();
    minOrder(root, res);
    return res;
}

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


/**
 * 非递归实现
 *
 * @param root
 * @return
 */
public static List<Integer> midOrder2(TreeNode root) {
    Stack<TreeNode> stack = new Stack<TreeNode>();
    List<Integer> list = new ArrayList<>();
    stack.add(root);
    while (!stack.isEmpty()) {
        TreeNode pop = stack.pop();
        if (pop != null) {
            stack.add(pop);
            stack.add(pop.left);
        } else if (!stack.isEmpty()) {
            TreeNode pop1 = stack.pop();
            list.add(pop1.val);
            stack.add(pop1.right);
        }
    }
    return list;
}


public static void main(String[] args) {
    TreeNode treeNode15 = new TreeNode(15, null, null);
    TreeNode treeNode7 = new TreeNode(7, null, null);
    TreeNode treeNode20 = new TreeNode(20, treeNode15, treeNode7);
    TreeNode treeNode9 = new TreeNode(9, null, null);
    TreeNode treeNode3 = new TreeNode(3, treeNode9, treeNode20);
    List<Integer> list = midOrder(treeNode3);
    list.stream().forEach(b -> {
        System.out.println(b);
    });
    System.out.println("=======");
    List<Integer> list2 = midOrder2(treeNode3);
    list.stream().forEach(b -> {
        System.out.println(b);
    });
}