LeetCode-in-Java

1095. Find in Mountain Array

Hard

(This problem is an interactive problem.)

You may recall that an array arr is a mountain array if and only if:

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

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

You cannot 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() <= 104
• 0 <= target <= 109
• 0 <= mountain_arr.get(index) <= 109

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() {}
 * }
 */
public class Solution {
    public int findInMountainArray(int target, MountainArray mountainArr) {
        int peakIndex = findPeak(mountainArr);
        if (target == mountainArr.get(peakIndex)) {
            return peakIndex;
        }
        int leftResult = findInPeakLeft(target, peakIndex, mountainArr);
        if (leftResult != -1) {
            return leftResult;
        }
        return findInPeakRight(target, peakIndex, mountainArr);
    }

    private int findPeak(MountainArray mountainArray) {
        int len = mountainArray.length();
        int left = 0;
        int right = len - 1;
        while (left < right) {
            int mid = left + (right - left) / 2;
            if (mountainArray.get(mid) < mountainArray.get(mid + 1)) {
                left = mid + 1;
            } else {
                right = mid;
            }
        }
        return left;
    }

    private int findInPeakLeft(int target, int peakIndex, MountainArray mountainArray) {
        int leftIndex = 0;
        int rightIndex = peakIndex - 1;
        while (leftIndex < rightIndex) {
            int midIndex = leftIndex + (rightIndex - leftIndex) / 2;
            if (target > mountainArray.get(midIndex)) {
                leftIndex = midIndex + 1;
            } else {
                rightIndex = midIndex;
            }
        }
        return target == mountainArray.get(leftIndex) ? leftIndex : -1;
    }

    private int findInPeakRight(int target, int peakIndex, MountainArray mountainArray) {
        int leftIndex = peakIndex + 1;
        int rightIndex = mountainArray.length() - 1;
        while (leftIndex < rightIndex) {
            int midIndex = leftIndex + (rightIndex - leftIndex) / 2;
            if (target < mountainArray.get(midIndex)) {
                leftIndex = midIndex + 1;
            } else {
                rightIndex = midIndex;
            }
        }
        return target == mountainArray.get(leftIndex) ? leftIndex : -1;
    }
}

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