Find in Mountain Array

LintCode Q 1095 - Peak Index in a Mountain Array

(This problem is an interactive problem.)
You may recall that an array A is a mountain array if and only if:

• A.length >= 3
• There exists some i with 0 < i < A.length - 1 such that:
  • A[0] < A[1] < ... A[i-1] < A[i]
  • A[i] > A[i+1] > ... > A[A.length - 1]

Given a mountain array mountainArr, return the minimum index such that mountainArr.get(index) == target. If such an index doesn’t exist, return -1.

You can’t access the mountain array directly. You may only access the array using a MountainArray interface:

• MountainArray.get(k) returns the element of the array at index k (0-indexed).
• MountainArray.length() returns the length of the array.
  Submissions making more than 100 calls to MountainArray.get will be judged Wrong Answer. Also, any solutions that attempt to circumvent the judge will result in disqualification.

Example 1: Input: array = [1,2,3,4,5,3,1], target = 3 ; Output: 2
Explanation: 3 exists in the array, at index=2 and index=5. Return the minimum index, which is 2.
Example 2: Input: array = [0,1,2,4,2,1], target = 3 ; Output: -1
Explanation: 3 does not exist in the array, so we return -1.

Constraints:

• 3 <= mountain_arr.length() <= 10000
• 0 <= target <= 10^9
• 0 <= mountain_arr.get(index) <= 10^9

Solution

// This is MountainArray's API interface.
// You should not implement it, or speculate about its implementation
interface MountainArray {
  public int get(int index) {}
  public int length() {}
}

Code:

public int findInMountainArray(int target, MountainArray ma) {
  int peakIndex = 0, len = mountainArr.length(), left = 0, right = len - 1;

  // find peak index
  while (left < right) {
    int mid = left + (right - left) / 2;
    if (ma.get(mid) < ma.get(mid + 1)) 
      left = mid + 1;
    else
      right = mid;
  }

  peakIndex = left;

  int peakNumber = ma.get(peakIndex);
  if (peakNumber == target) return peakIndex;
  if (peakNumber < target) return -1;

  int res = helper(ma, target, 0, peakIndex - 1, 1);
  if (res != -1) return res;

  return helper(ma, target, peakIndex + 1, len - 1, -1);
}

private int helper (MountainArray mountainArr, int target, int left, int right, int indicator) {
  while (left <= right) {
    int mid = (left + right) / 2;
    if (mountainArr.get(mid) == target)
      return mid;
    else if (mountainArr.get(mid) * indicator> target * indicator)
      right = mid - 1;
    else 
      left = mid + 1;
  }
  
  return -1;
}