题解 | #括号生成# 递归 + 条件补全

import java.util.*;


public class Solution {
    public ArrayList<String> dpBuild(int n) {
        if (n == 1) {
            return new ArrayList<String>(
                       Arrays.asList("()")
                   );
        } else if (n == 2) {
            return new ArrayList<String>(
                       Arrays.asList("()()", "(())")
                   );
        } else {
            Set<String> result = new HashSet<String>();
            for(int i = 1; i <= n-1; i++){
                ArrayList<String> a = dpBuild(i);
                ArrayList<String> b = dpBuild(n-i);
                StringBuilder pbuider = new StringBuilder();
                StringBuilder lbuider = new StringBuilder();
                int k = i;
                while(k-- > 0){
                    pbuider.append("(");
                    lbuider.append(")");
                }
                String pre = pbuider.toString();
                String last = lbuider.toString();

                for(String aStr:a){
                    for(String bstr:b){
                        result.add(aStr + bstr);
                        result.add(bstr + aStr);
                        result.add(pre + bstr + last);
                    }
                }
            }
           
            return new ArrayList<>(result);
        }
    }
    public ArrayList<String> generateParenthesis (int n) {
        // write code here
        ArrayList<String> result = dpBuild(n);
        return result;
    }
}

模拟出单侧聚集 (((((((((((((((( 类似的条件