leetcode 22. Generate Parentheses

生成多重括号，使用回溯法试探所有组合

 function generateParenthesis(n) {
      // Write your code here
      var res = [], condidate = []
      function backtrack(n, left, right) {
        if (condidate.length == n * 2) {
          res.push(condidate.join(''));
        }
        if (left < n) { //Keep 塞左括号
          condidate.push('(');
          backtrack(n, left + 1, right);
          condidate.pop();
        }
        if (right < left) { //避免出现右括号多余左括号的情况
          condidate.push(')');
          backtrack(n, left, right + 1);
          condidate.pop();
        }
      }
      backtrack(n, 0, 0);
      return res;
    }