[leetcode]Generate Parentheses

简单题。DFS。还是用Java写函数麻烦了。一开始把left<n的条件忘了。

import java.util.ArrayList;



public class Solution {

    public ArrayList<String> generateParenthesis(int n) {

        // Start typing your Java solution below

        // DO NOT write main() function

        ArrayList<String> arr = new ArrayList<String>();

    	ArrayList<Character> chars = new ArrayList<Character>();

    	dfs(arr, 0, 0, chars, n);

    	return arr;

    }

    

    private void dfs(ArrayList<String> arr, int left, int right, ArrayList<Character> chars, int n)

    {

    	if (left + right + 1 == n * 2)

    	{

    		if (left < right) {

    			// chars.add('(')  - impossible

    			// chars.remove(chars.size() - 1);

    		}

    		else

    		{

    			chars.add(')');

    			StringBuilder sb = new StringBuilder();

    			for (char c : chars) {

    				sb.append(c);

    			}

    			arr.add(sb.toString());

    			chars.remove(chars.size() - 1);

    		}

    	}

    	else

    	{

    		if (left > right) {

    			chars.add(')');

    			dfs(arr, left, right + 1, chars, n);

    			chars.remove(chars.size() - 1);

    		}

    		if (left < n)

    		{

	    		chars.add('(');

	    		dfs(arr, left + 1, right, chars, n);

	    		chars.remove(chars.size() - 1);

    		}

    	}

    }

}