Lexicographically Largest Mountain List - Amazon Top Interview Questions
Problem Statement :
You are given three positive integers n, lower, and upper. You want to create a list of length n that is strictly increasing and then strictly decreasing and all the numbers are between [lower, upper], inclusive. Each of the increasing and decreasing parts should be non-empty. Return the lexicographically largest list possible, or the empty list if it's not possible. Constraints 3 ≤ n ≤ 100,000 1 ≤ lower ≤ upper < 2 ** 31 Example 1 Input n = 5 lower = 2 upper = 6 Output [5, 6, 5, 4, 3] Explanation Note that [6, 5, 4, 3, 2] is not valid since the strictly increasing part has to be non-empty. Example 2 Input n = 5 lower = 90 upper = 92 Output [90, 91, 92, 91, 90] Example 3 Input n = 6 lower = 3 upper = 5 Output [] Explanation It's impossible to make a strictly increasing then decreasing list of size 6 here.
Solution :
Solution in C++ :
vector<int> solve(int n, int lower, int upper) {
deque<int> d;
int num = upper;
for (int i = 0; i < n - 1; ++i) {
if (num < lower) break;
d.push_back(num--);
}
int right = d.size();
num = upper - 1;
for (int i = 0; i < n - right; ++i) {
if (num < lower) break;
d.push_front(num--);
}
if (d.size() != n) return {};
vector<int> res;
for (int i = 0; i < d.size(); ++i) res.push_back(d[i]);
return res;
}
Solution in Java :
import java.util.*;
class Solution {
public int[] solve(int N, int lower, int upper) {
int[] ans = new int[N];
if (N < 3 || lower == upper) {
ans = new int[] {};
} else if (N <= upper - lower + 2) {
ans[0] = upper - 1;
for (int i = 1; i < N; i++) ans[i] = upper - i + 1;
} else if (N <= 2 * (upper - lower) + 1) {
int cur = lower;
int dir = 1;
for (int i = N - 1; i >= 0; i--) {
ans[i] = cur;
if (cur == upper)
dir = -1;
cur += dir;
}
} else {
ans = new int[] {};
}
return ans;
}
}
Solution in Python :
class Solution:
def solve(self, n, lower, upper):
i = j = upper - 1
n -= 3
while j > lower and n:
j -= 1
n -= 1
while i > lower and n:
i -= 1
n -= 1
if n or lower == upper:
return []
return list(range(i, upper)) + list(range(upper, j - 1, -1))
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