Array216CombinationSum3

Backtracking: abandons each partial candidate c (“backtracks”) as soon as it determines that c cannot possibly be completed to a valid solution.

思路一:

只能理解https://discuss.leetcode.com/topic/37962/fast-easy-java-code-with-explanation 这个讲解, 以后再做
todo:

https://discuss.leetcode.com/topic/26351/simple-and-clean-java-code-backtracking
竟然忘了这个. 看来定义的形状不一样是不能随便加的
List<Integer> li = new ArrayList<Integer>(list);