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],
]

/**
 * @param {number} n
 * @param {number} k
 * @return {number[][]}
 */

// return an array of k numbers with 1 ~ n

var ans = []
  , tmp = [];

function dfs(next, n, num, k) {
  if (num === k) {
    var res = tmp.map(function(item) {
      return item;
    });

    ans.push(res);
    return;
  }

  for (var i = next; i <= n; i++) {
    tmp.push(i);
    dfs(i + 1, n, num + 1, k);
    tmp.pop();
  }
}

var combine = function(n, k) {
  ans = []; 
  // select the 1st number
  for (var i = 1; i <= n; i++) {
    tmp[0] = i;
    dfs(i + 1, n, 1, k);
  }

  return ans;
};

Compare Version Numbers

Compare two version numbers version1 and version2.
If version1 > version2 return 1, if version1 < version2 return -1, otherwise return 0.

You may assume that the version strings are non-empty and contain only digits and the . character.
The . character does not represent a decimal point and is used to separate number sequences.
For instance, 2.5 is not “two and a half” or “half way to version three”, it is the fifth second-level revision of the second first-level revision.

Here is an example of version numbers ordering:

0.1 < 1.1 < 1.2 < 13.37

/**
 * @param {string} version1
 * @param {string} version2
 * @return {number}
 */
var compareVersion = function(version1, version2) {
  var a = version1.split('.');
  var b = version2.split('.');

  var len1 = a.length;
  var len2 = b.length;
  for (var i = 0, len = Math.max(len1, len2); i < len; i++) {
    var item1 = i < len1 ? +a[i] : 0;
    var item2 = i < len2 ? +b[i] : 0;

    if (item1 > item2)
      return 1;

    if (item1 < item2)
      return -1;
  }
};

Complex Number Multiplication

Given two strings representing two complex numbers.

You need to return a string representing their multiplication. Note $i^2 = -1$ according to the definition.

Example 1:

Input: "1+1i", "1+1i"
Output: "0+2i"
Explanation: (1 + i) * (1 + i) = 1 + i2 + 2 * i = 2i, and you need convert it to the form of 0+2i.

Example 2:

Input: "1+-1i", "1+-1i"
Output: "0+-2i"
Explanation: (1 - i) * (1 - i) = 1 + i2 - 2 * i = -2i, and you need convert it to the form of 0+-2i.

Note

• The input strings will not have extra blank.
• The input strings will be given in the form of a+bi, where the integer a and b will both belong to the range of [-100, 100]. And the output should be also in this form.

class Solution(object):
    def complexNumberMultiply(self, a, b):
        """
        :type a: str
        :type b: str
        :rtype: str
        """
        a = [int(i) for i in a[:-1].split('+')]
        b = [int(i) for i in b[:-1].split('+')]
        c = a[0] * b[0] - a[1] * b[1]
        d = a[0] * b[1] + a[1] * b[0]
        return str(c) + '+' + str(d) + 'i'