* [1. 给表达式加括号](#1-给表达式加括号)
* [2. 不同的二叉搜索树](#2-不同的二叉搜索树)
# 1. 给表达式加括号
[241. Different Ways to Add Parentheses (Medium)](https://leetcode.com/problems/different-ways-to-add-parentheses/description/)
```html
Input: "2-1-1".
((2-1)-1) = 0
(2-(1-1)) = 2
Output : [0, 2]
```
```java
public List diffWaysToCompute(String input) {
List ways = new ArrayList<>();
for (int i = 0; i < input.length(); i++) {
char c = input.charAt(i);
if (c == '+' || c == '-' || c == '*') {
List left = diffWaysToCompute(input.substring(0, i));
List right = diffWaysToCompute(input.substring(i + 1));
for (int l : left) {
for (int r : right) {
switch (c) {
case '+':
ways.add(l + r);
break;
case '-':
ways.add(l - r);
