[Algorithm] Recursion

유의할 점

• base case는 필수 (The base case returns a value without making any subsequent recursive call.)
• return statement

Recursive Factorial

// for loop 
const factorial = (n) => {
  let total = 1;
  for(let i=n; i>1; i--) {
    total *= i;
  }
  return total;
}

// recursion
const factorial = (n) => {
  if(n === 1) return 1;
  return n * factorial(n-1)
}

Helper method recursion

패턴

const outer = (input) => {
  let outerScopeVariable = [];
  
  const helper = (helperInput) => {
    helper(helperInput--);
  } 
    
  helper(input);
  return outerScopeVariable;
}

예시

function collectOddValues(arr){
    
    let result = [];

    function helper(helperInput){
        if(helperInput.length === 0) {
            return;
        }
        
        if(helperInput[0] % 2 !== 0){
            result.push(helperInput[0])
        }
        
        helper(helperInput.slice(1))
    }
    
    helper(arr)

    return result;
}

collectOddValues([1,2,3,4,5,6,7,8,9])