911. Online Election

In an election, the i-th vote was cast for persons[i] at time times[i].

Now, we would like to implement the following query function: TopVotedCandidate.q(int t) will return the number of the person that was leading the election at time t.

Votes cast at time t will count towards our query. In the case of a tie, the most recent vote (among tied candidates) wins.

Example 1:

Input: ["TopVotedCandidate","q","q","q","q","q","q"], [[[0,1,1,0,0,1,0],[0,5,10,15,20,25,30]],[3],[12],[25],[15],[24],[8]]
Output: [null,0,1,1,0,0,1]
Explanation:
At time 3, the votes are [0], and 0 is leading.
At time 12, the votes are [0,1,1], and 1 is leading.
At time 25, the votes are [0,1,1,0,0,1], and 1 is leading (as ties go to the most recent vote.)
This continues for 3 more queries at time 15, 24, and 8.

Note:

1. 1 <= persons.length = times.length <= 5000
2. 0 <= persons[i] <= persons.length
3. times is a strictly increasing array with all elements in [0, 10^9].
4. TopVotedCandidate.q is called at most 10000 times per test case.
5. TopVotedCandidate.q(int t) is always called with t >= times[0].

Solutions (Rust)

1. Binary Search

use std::collections::HashMap;

struct TopVotedCandidate {
    winners: Vec<(i32, i32)>,
}

/**
 * `&self` means the method takes an immutable reference.
 * If you need a mutable reference, change it to `&mut self` instead.
 */
impl TopVotedCandidate {
    fn new(persons: Vec<i32>, times: Vec<i32>) -> Self {
        let mut counter = HashMap::new();
        let mut max = 0;
        let mut winner = 0;
        let mut winners = vec![];

        for (p, t) in persons.into_iter().zip(times.into_iter()) {
            let c = counter.entry(p).or_insert(0);
            *c += 1;
            if *c >= max {
                max = *c;
                winner = p;
            }
            winners.push((t, winner));
        }

        Self { winners }
    }

    fn q(&self, t: i32) -> i32 {
        match self.winners.binary_search_by_key(&t, |&(time, _)| time) {
            Ok(i) => self.winners[i].1,
            Err(i) => self.winners[i - 1].1,
        }
    }
}

/**
 * Your TopVotedCandidate object will be instantiated and called as such:
 * let obj = TopVotedCandidate::new(persons, times);
 * let ret_1: i32 = obj.q(t);
 */