[JS/알고리즘] Find First and Last Position of Element in Sorted Array

BST

이미 sorted 된 데이터의 경우, BST를 이용해 O(log n)의 시간복잡도로 타겟을 찾을 수 있다.

 

34. Find First and Last Position of Element in Sorted Array

Given an array of integers nums sorted in ascending order, find the starting and ending position of a given target value.

Your algorithm's runtime complexity must be in the order of O(log n).

If the target is not found in the array, return [-1, -1].

Example 1:
Input: nums = [5,7,7,8,8,10], target = 8
Output: [3,4]

Example 2:
Input: nums = [5,7,7,8,8,10], target = 6
Output: [-1,-1]

 

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const searchRange = function (nums, target) {
  const { length } = nums;
  let firstIndex = 0;
  let lastIndex = length - 1;

  while (firstIndex <= lastIndex) {
    const midIndex = Math.floor((firstIndex + lastIndex) / 2);

    if (nums[midIndex] === target) {
      firstIndex = midIndex;
      lastIndex = midIndex;
      break;
    }

    if (nums[midIndex] < target) {
      //타겟이 더 크므로 뒷부분에서 탐색
      firstIndex = midIndex + 1;
    } else {
      lastIndex = midIndex - 1;
    }
  }

  // while문을 끝까지 순회 했으나 target을 못찾은 경우
  if (firstIndex > lastIndex) {
    return [-1, -1];
  }

  //중간에 break되어서 나왔을 경우, 앞뒤로 같은 값 있는지 탐색
  while (nums[firstIndex - 1] === target) firstIndex--;
  while (nums[lastIndex + 1] === target) lastIndex++;

  return [firstIndex, lastIndex];
};

출처 : leetcode.com/problems/find-first-and-last-position-of-element-in-sorted-array/