上一層的函式還沒執行完就呼叫下一層，因此，當達到遞迴終止條件時，首先return的是最底層呼叫的函式

int Sum(int n)
{
    if (n <= 1)                  //#1
        return n;                //#2
    return n+Sum(n - 1);         //#3
}

若n=10，則return 10 + Sum(9),接著return 10 + 9 + Sum(8)... 
當n=1時，最底層return 1 給倒數第二層，
倒數第二層return 2 + 1 給倒數第三層...
最後返回的結果就是 10 + 9 + 8 + ... + 1 = 55

java遞迴演算法遍歷二叉樹：

public class TreeNode {
    public int data;
    public TreeNode leftChild;
    public TreeNode rightChild;


    public TreeNode(int data) {
        this.data = data;
    }

    /**
     * 建立二叉樹
     *
     * @param list
     * @return
     */
    public static TreeNode createBinaryTree(LinkedList<Integer> list) {
        TreeNode node = null;
        if (list == null || list.isEmpty()) {
            return null;
        }
        Integer data = list.removeFirst();
        if (data != null) {
            node = new TreeNode(data);
            node.leftChild = createBinaryTree(list);
            node.rightChild = createBinaryTree(list);
        }
        return node;
    }

    public static void main(String[] args) {

        LinkedList<Integer> inputList1 = new LinkedList<>
                (Arrays.asList(1, 2, 3, null, null, 4, 5, null, 6, null, null, 7, null, null));
        TreeNode treeNode = createBinaryTree(inputList1);
        System.out.println("前序遍歷：");
        preOrderTraveral(treeNode);
        System.out.println("\n" + "中序遍歷：");
        inOrderTraveral(treeNode);
        System.out.println("\n" + "後序遍歷：");
        postOrderTraveral(treeNode);

    }


    /**
     * 前序遍歷
     *
     * @param node
     */
    public static void preOrderTraveral(TreeNode node) {
        if (node == null) {
            return;
        }
        System.out.print(node.data + " ");
        preOrderTraveral(node.leftChild);
        preOrderTraveral(node.rightChild);

    }


    /**
     * 中序遍歷
     *
     * @param node
     */
    public static void inOrderTraveral(TreeNode node) {
        if (node == null) {
            return;
        }
        inOrderTraveral(node.leftChild);
        System.out.print(node.data + " ");
        inOrderTraveral(node.rightChild);

    }


    /**
     * 後續遍歷
     *
     * @param node
     */
    public static void postOrderTraveral(TreeNode node) {
        if (node == null) {
            return;
        }
        postOrderTraveral(node.leftChild);
        postOrderTraveral(node.rightChild);
        System.out.print(node.data + " ");

    }
}