博客

22. Generate Parentheses

Given n pairs of parentheses, write a function to generate all combinations of well-formed parentheses.

For example, given n = 3, a solution set is:

[
  "((()))",
  "(()())",
  "(())()",
  "()(())",
  "()()()"
]

code:

class Solution {
     public List<String> generateParenthesis(int n) {
        List<String> list = new ArrayList<String>();
        backtrack(list, "", 0, 0, n);
        return list;
    }

    public void backtrack(List<String> list, String str, int open, int close, int max){

        if(str.length() == max*2){
            list.add(str);
            return;
        }

        if(open < max)
            backtrack(list, str + "(", open+1, close, max);
        if(close < open)
            backtrack(list, str+")", open, close+1, max);
    }
}

77. Combinations

Given two integers n and k, return all possible combinations of k numbers out of 1 … n.

For example,

If n = 4 and k = 2, a solution is:

[
  [2,4],
  [3,4],
  [2,3],
  [1,2],
  [1,3],
  [1,4],
]

code:

class Solution {
    public List<List<Integer>> combine(int n, int k) {
        List<List<Integer>> res = new LinkedList<List<Integer>>();
        List<Integer> comb = new LinkedList<>();
        dfs(res, comb, n, k, 1);
        return res;
    }

    public void dfs(List<List<Integer>> res, List<Integer> comb, int n, int k, int start) {
        if(comb.size() == k) {
            res.add(new LinkedList<Integer>(comb));
            return;
        }
        for(int i=start; i<=n; i++) {
            comb.add(i);
            dfs(res, comb, n, k, i+1);
            comb.remove(comb.size()-1);
        }
    }
}

特别注意

：

res.add(new LinkedList<Integer>(comb));

dfs(res, comb, n, k, i+1);