An element x of an integer array arr of length m is dominant if more than half the elements of arr have a value of x. You are given a 0-indexed integer array nums of length n with one dominant element. You can split nums at an index i into two arra

minimumIndex.js

/**
 * Finds the minimum index to split the array such that both subarrays
 * have the same dominant element.
 * 
 * @param {number[]} nums - The input array of integers.
 * @return {number} - The minimum index of a valid split, or -1 if no valid split exists.
 */
var minimumIndex = function(nums) {
    const n = nums.length;

    // Step 1: Identify the dominant element in the array.
    // The "dominant" element is the one that appears more than half the time.
    let dominant = 0; // Candidate for the dominant element.
    let count = 0;    // Tracks occurrences of the candidate.
    for (let num of nums) {
        if (num === dominant) {
            count += 1; // Increment if the current number matches the candidate.
        } else if (count === 0) {
            dominant = num; // Update candidate when no current count.
            count = 1;
        } else {
            count -= 1; // Decrement count when num differs from the candidate.
        }
    }

    // Step 2: Count the total occurrences of the dominant element in the array.
    let second_half = 0; // Total count of the dominant element.
    for (let i = 0; i < n; i++) {
        if (nums[i] === dominant) {
            second_half += 1;
        }
    }

    // Step 3: Find the smallest valid split index.
    let first_half = 0; // Count of the dominant element in the left subarray.
    for (let i = 0; i < n; i++) {
        if (nums[i] === dominant) {
            first_half += 1;  // Increment count in the left subarray.
            second_half -= 1; // Decrement count in the right subarray.

            // Check if both subarrays have the dominant element as dominant.
            if (first_half > (i + 1) / 2 && second_half > (n - 1 - i) / 2) {
                return i; // Valid split found at index i.
            }
        }
    }

    // If no valid split is found, return -1.
    return -1;
};