C1_69_NParentheses.java

package com.interview.algorithms.general;

import java.util.ArrayList;
import java.util.List;

/**
 * Created_By: stefanie
 * Date: 14-10-26
 * Time: 下午6:26
 */
public class C1_69_NParentheses {
    public static List<String> find(int n){
        List<String> sol = new ArrayList<String>();
        find(sol, n, n, new char[2 * n], 0);
        return sol;
    }

    private static void find(List<String> sol, int left, int right, char[] cur, int count){
        if(left < 0 || right < left) return;
        if(left == 0 && right == 0) sol.add(String.copyValueOf(cur));
        else {
            if(left > 0){
                cur[count] = '(';
                find(sol, left - 1, right, cur, count + 1);
            }
            if(right > left){
                cur[count] = ')';
                find(sol, left, right - 1, cur, count + 1);
            }
        }
    }

    /*
       Below is an alternative implementation
     */
    public void print(int N) {
        for(String s : this.generate("(", 1, 0, N))
            System.out.println(s);
    }

    private List<String> generate(String prefix, int left, int right, int N) {
        List<String> result = new ArrayList<String>();
        if(left == N) {
            String combination = new String(prefix);
            while(right < N) {
                combination += ")";
                right ++;
            }
            result.add(combination);
        } else {
            if(right < N && right < left)
                result.addAll(generate(prefix + ")", left, right + 1, N));
            result.addAll(generate(prefix + "(", left + 1, right, N));
        }
        return result;
    }
}