题目链接

22.括号生成

class Solution {List<String> res = new ArrayList<>();StringBuilder path = new StringBuilder();void backtracking(int n, int left, int right) {if (left + right == 2 * n) {res.add(path.toString());return;}if (left < n) {path.append('(');backtracking(n, left + 1, right);path.deleteCharAt(path.length() - 1);}if (right < left) {path.append(')');backtracking(n, left, right + 1);path.deleteCharAt(path.length() - 1);}}public List<String> generateParenthesis(int n) {backtracking(n, 0, 0);return res;}
}

小结：和前面的回溯问题不同，不是靠索引或位置推进，而是用左右括号计数器推进。核心约束是：1. 左括号数不超过n ；2. 右括号数不超过左括号数。