freeCodeCamp/curriculum/challenges/japanese/10-coding-interview-prep/project-euler/problem-206-concealed-square.md

id	title	challengeType	forumTopicId	dashedName
5900f43a1000cf542c50ff4d	問題 206: 隠された平方数	5	301847	problem-206-concealed-square

--description--

「_」は 1 つの数を表すものとします。2 乗すると 1_2_3_4_5_6_7_8_9_0 の形になる唯一の正の整数を求めなさい。

--hints--

concealedSquare() は 1389019170 を返す必要があります。

assert.strictEqual(concealedSquare(), 1389019170);

--seed--

--seed-contents--

function concealedSquare() {

  return true;
}

concealedSquare();

--solutions--

// Check if n**2 matches the pattern
function squareMatchs(n) {
  // Need BigInt due to size of values
  let nSquared = (BigInt(n) * BigInt(n)).toString();

  // Check if digits match pattern
  for (let i = 1; i <= 9; i++) {
    if (nSquared[2 * (i - 1)] != i) return false;
  }
  return true;
}

// Find integer whose square matches the pattern
function concealedSquare() {
  // Set bounds based upon max and min candidates
  const minSquareRoot = Math.floor(Math.sqrt(10203040506070809) / 10) * 10;
  const maxSquareRoot = Math.ceil(Math.sqrt(19293949596979899) / 10) * 10;

  for (let x = maxSquareRoot; x >= minSquareRoot; x -= 10) {
    // Note: 3*3 = 9 and 7*7 = 49 are only trailing digits
    //       that can produce 9 as trailing digit in square
    if (squareMatchs(x + 3)) return (x + 3)*10;
    if (squareMatchs(x + 7)) return (x + 7)*10;
  }
  return -1;
}