Recursion

function generateParenthesis(n: number): string[] {
  const result: string[] = [];
  function backtrack(current: string, open: number, close: number) {
    if (current.length === 2 * n) {
      result.push(current)
      return
    }
 
    if (open < n) {
      backtrack(current + "(", open+1, close)
    }
 
    if (close < open ) {
      backtrack(current + ")", open, close+1)
    }
  }
 
  backtrack("", 0 , 0)
 
  return result
}
 
console.log('generateParenthesis(20):', generateParenthesis(5))

 
function combinationSum(candidates: number[], target: number): number[][] {
  const result: number[][] = []
 
  function helper(current: number[], start: number, target: number) {
    if (target < 0) {
      return
    }
 
    if (target === 0) {
      result.push([...current])
      return
    }
 
    for (let i = start; i < candidates.length; i++) {
      current.push(candidates[i])
      helper(current, i, target - candidates[i])
      current.pop()
    }
  }
 
  helper([], 0, target)
 
  return result
}
 
console.log('combinationSum([2,3,6,7], 7):', combinationSum([2,3,6,7], 7))