[Algorithm] Search element in a circular sorted array

Posted 2021-11-07 answer1215

function findInCircularlySortedAry (ary = [], target) {
  if (ary && ary.length === 0) {
    return -1;
  }
  
  if (ary.length === 1) {
      return ary[0] === target ? 0 : -1;
  }
  
  let low = 0,
      high = ary.length - 1;
  
  while(low <= high) {
    let mid = Math.floor((low + high) / 2);
    // case 1: target === middle item, return found
    if (ary[mid] === target) {
       return mid;
    }
    // To find which parts (left or right) is sorted
    // case 2: if middle < high, mean from middle to high is sorted
    if (ary[mid] < ary[high]) {
       if (target > ary[mid] && target <= ary[high]) {
           low = mid + 1;
       } else {
         high = mid - 1;
       }
    } 
    // case 3: if low < middle, mean from low to middle is sorted
    else {
      if (target >= ary[low] && target < ary[mid]) {
        high = mid -1;
      } else {
        low = mid + 1;
      }
    } 
  }
  
  return -1;
}


const data = [12,14,18,21,3,6,8,9];
const res = findInCircularlySortedAry(data,18); // 2
console.log(res);

We don‘t need to