Say you have an array for which the ith element is the price of a given stock on day i.

maxProfit.java

/*
Say you have an array for which the ith element is the price of a given stock on day i.

If you were only permitted to complete at most one transaction (ie, buy one and sell one share of the stock), design an algorithm to find the maximum profit.
*/
import java.util.*;
public class Solution {
    public class Stock implements Comparable<Stock>{
        public int index;
        public int price;
        public Stock(int i, int p) {
            this.index = i;
            this.price = p;
        }
        
        @Override
        public int compareTo(Stock other) {
            return Integer.valueOf(other.price).compareTo(price);
        }
    }
 
    public int maxProfit(int[] prices) {
        // Start typing your Java solution below
        // DO NOT write main() function
        
        int size = prices.length;
        int diff = 0;
        int max_diff = 0;

        PriorityQueue<Stock> queue = new PriorityQueue<Stock>();
        
        for(int i = 0; i < size; i++) {
            queue.offer(new Stock(i, prices[i]));
        }
        
        Stock max_price = queue.poll();
        for(int i = 0; i < size; i++) {
            while(max_price.index < i) {
                max_price = queue.poll();
            }
            
            diff = max_price.price - prices[i];
            if(diff > max_diff) {
                max_diff = diff;
            }
        }
        
        return max_diff;

    }
}