77. Combinations

Problem:

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

Example:

Input: n = 4, k = 2
Output:
[
  [2,4],
  [3,4],
  [2,3],
  [1,2],
  [1,3],
  [1,4],
]

Solution:

Basic DFS + Backtracking.

/**
 * @param {number} n
 * @param {number} k
 * @return {number[][]}
 */
let combine = function (n, k) {
  const result = [];
  _combine(1, [], n, k, result);
  return result;
};

function _combine(cur, path, n, k, result) {
  if (path.length === k) {
    return result.push(path.slice());
  }

  while (cur <= n) {
    path.push(cur);
    _combine(++cur, path, n, k, result);
    path.pop();
  }
}

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