LeetCode 2031: Count Subarrays With More Ones Than Zeros Solution

Master LeetCode problem 2031 (Count Subarrays With More Ones Than Zeros), a medium challenge, with our optimized solutions in Java, C++, and Python. Explore detailed explanations, test your code in our interactive editor, and prepare for coding interviews.

2031. Count Subarrays With More Ones Than Zeros

Medium

Array Binary Search Divide and Conquer Binary Indexed Tree Segment Tree Merge Sort Ordered Set

Problem Explanation

Explanation

To solve this problem, we can iterate through the array and keep track of the running sum of ones and zeros encountered so far. For each index i, we calculate diff = ones - zeros where ones is the number of ones encountered so far and zeros is the number of zeros encountered so far. If diff > 0, it means we have more ones than zeros in the subarray ending at index i. We can increment the count of valid subarrays by diff.

We can use a hashmap to keep track of the running sum of ones and zeros encountered so far. We initialize the hashmap with a key-value pair of (0,1) to handle the case when the running sum is 0 initially.

Time complexity: O(n) where n is the length of the input array. Space complexity: O(n) for the hashmap.

Solution Code

import java.util.HashMap;

class Solution {
    public int countSubarrays(int[] nums) {
        int ones = 0, zeros = 0, count = 0;
        HashMap<Integer, Integer> map = new HashMap<>();
        map.put(0, 1);

        for (int num : nums) {
            if (num == 1) {
                ones++;
            } else {
                zeros++;
            }

            int diff = ones - zeros;
            count += map.getOrDefault(diff, 0);
            map.put(diff, map.getOrDefault(diff, 0) + 1);
        }

        return count;
    }
}

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