[UCSD白板题] Majority Element

Problem Introduction

An element of a sequence of length \(n\) is called a majority element if it appears in the sequence strictly more than \(n/2\) times.

Problem Description

Task.The goal in this code problem is to check whether an input sequence contains a majority element.

Input Format.The first line contains an integer \(n\), the next one contains a sequence of \(n\) non-negative integers \(a_0,a_1,\cdots,a_{n-1}\).

Constraints.\(1 \leq n \leq 10^5; 0 \leq a_i \leq 10^9\) for all \(0 \leq i < n\).

Output Format.Ouput 1 if the sequence contains a majority element and 0 otherwise.

Sample 1.
Input:

5
2 3 9 2 2

Output:

1

Sample 2.
Input:

4
1 2 3 4

Output:

0

Sample 3.
Input:

4
1 2 3 1

Output:

0

Solution

# Uses python3
import sys

def get_majority_element(a, left, right):
    if left == right:
        return -1
    if left + 1 == right:
        return a[left]
    mid = left + (right - left) // 2
    majority_left = get_majority_element(a, left, mid)
    majority_right = get_majority_element(a, mid, right)
    b = a[left:right]
    threshold = len(b) // 2
    if b.count(majority_left) > threshold:
        return majority_left
    if b.count(majority_right) > threshold:
        return majority_right
    return -1

if __name__ == '__main__':
    input = sys.stdin.read()
    n, *a = list(map(int, input.split()))
    if get_majority_element(a, 0, n) != -1:
        print(1)
    else:
        print(0)