22. Generate Parentheses
backtrack 再熟悉一下
class Solution { List<String> res = new ArrayList<>(); public List<String> generateParenthesis(int n) { dfs("", n, n, n); return res; } public void dfs(String s, int n1, int n2, int n){ if(n1 == 0 && n2 == 0) { res.add(s); return; } if(n1 > 0) { // s = s + "("; 不能这样加 会改变现有的s dfs(s + "(", n1 - 1, n2, n); } if(n2 > 0 && n2 > n1) { // s = s + ")"; dfs(s + ")", n1, n2-1 ,n); } } }