等差数列中缺失的数字

/*
 * Problem: Missing Number In Arithmetic Progression (Leetcode 1228)
 * Given an array that represents elements of an arithmetic progression in order,
 * one element is missing in the progression. Find the missing element.
 * 
 * Approach using Java Streams:
 * 1. Calculate the expected difference (common difference) of the progression.
 * 2. Use IntStream to iterate through the array indices and find where the difference
 *    between consecutive elements is not equal to the expected difference.
 * 3. If a mismatch is found, return the missing number by adding the expected difference
 *    to the previous element.
 */

import java.util.stream.IntStream;

public class MissingNumberInAPStream {
    public int missingNumber(int[] arr) {
        int n = arr.length;
        int totalDifference = arr[n-1] - arr[0];
        int commonDifference = totalDifference / n;
        
        return IntStream.range(0, n - 1)
                        .filter(i -> arr[i + 1] - arr[i] != commonDifference)
                        .mapToObj(i -> arr[i] + commonDifference)
                        .findFirst()
                        .orElse(-1);
    }
}