Buy Low, Sell High

#include <iostream>
#include <stdint.h>
 
using namespace std;
 
int64_t prices[500000];
 
int main() {
    int n;
    cin >> n;
 
    for (int i = 0; i < n; i++) {
        cin >> prices[i];
    }
    
    int start = 0;
    int64_t best = 0;
    int best_pos = 0;
 
    for (int i = 0; i < n; i++) {
        if (prices[i] < prices[start]) {
            start = i;
        }
        
        int64_t p = prices[i] - prices[start];
        if (p > best) {
            best = p;
            best_pos = i;
        }
    }
    
    int bstart = start;
    int bend = best_pos;
    int64_t bbest = best;
    
    //
    start = 0;
    best = 0;
    best_pos = 0;
    
    for (int i = 0; i <= bstart; i++) {
        if (prices[i] < prices[start]) {
            start = i;
        }
        
        int64_t p = prices[i] - prices[start];
        if (p > best) {
            best = p;
            best_pos = i;
        }
    }
    
    int64_t abest = best;
    
    //
    start = bend;
    best = 0;
    best_pos = bend;
 
    for (int i = bend; i < n; i++) {
        if (prices[i] < prices[start]) {
            start = i;
        }
        
        int64_t p = prices[i] - prices[start];
        if (p > best) {
            best = p;
            best_pos = i;
        }
    }
    
    int64_t ebest = best;
    
    //
    start = bend;
    best = 0;
    best_pos = bend;
 
    for (int i = bend; i > bstart; i--) {
        if (prices[i] < prices[start]) {
            start = i;
        }
        
        int64_t p = prices[i] - prices[start];
        if (p > best) {
            best = p;
            best_pos = i;
        }
    }
    
    int64_t drop = best;
    
    cout << max(bbest + abest, max(bbest + ebest, bbest + drop)) << endl;
    
}

Test 1

Verdict: ACCEPTED

Test 2

Verdict: ACCEPTED

Test 3

Verdict: ACCEPTED

Test 4

Verdict: WRONG ANSWER

Test 5

Verdict: ACCEPTED

Test 6

Verdict: ACCEPTED

Test 7

Verdict: ACCEPTED

Test 8

Verdict: ACCEPTED

Test 9

Verdict: ACCEPTED