tag:blogger.com,1999:blog-91611294025112880012024-09-26T15:31:33.328-07:00BhaaratMaaBhaarathttp://www.blogger.com/profile/08379295257807934343noreply@blogger.comBlogger2125tag:blogger.com,1999:blog-9161129402511288001.post-82045506934739163642021-06-12T23:53:00.002-07:002021-06-13T00:05:02.382-07:00 [Leetcode] 871. Minimum Number of Refueling Stops<b>Problem statement:</b>
A car travels from a starting position to a destination which is target miles east of the starting position.
Along the way, there are gas stations. Each station[i] represents a gas station that is station[i][0] miles east of the starting position, and has station[i][1] liters of gas.
The car starts with an infinite tank of gas, which initially has startFuel liters of fuel in it. It uses 1 liter of gas per 1 mile that it drives.
When the car reaches a gas station, it may stop and refuel, transferring all the gas from the station into the car.
What is the least number of refueling stops the car must make in order to reach its destination? If it cannot reach the destination, return -1.
Note that if the car reaches a gas station with 0 fuel left, the car can still refuel there. If the car reaches the destination with 0 fuel left, it is still considered to have arrived.
Example 1:
Input: target = 1, startFuel = 1, stations = []
Output: 0
Explanation: We can reach the target without refueling.
Example 2:
Input: target = 100, startFuel = 1, stations = [[10,100]]
Output: -1
Explanation: We can't reach the target (or even the first gas station).
Example 3:
Input: target = 100, startFuel = 10, stations = [[10,60],[20,30],[30,30],[60,40]]
Output: 2
Explanation:
We start with 10 liters of fuel.
We drive to position 10, expending 10 liters of fuel. We refuel from 0 liters to 60 liters of gas.
Then, we drive from position 10 to position 60 (expending 50 liters of fuel),
and refuel from 10 liters to 50 liters of gas. We then drive to and reach the target.
We made 2 refueling stops along the way, so we return 2.
Note:
1 <= target, startFuel, stations[i][1] <= 10^9
0 <= stations.length <= 500
0 < stations[0][0] < stations[1][0] < ... < stations[stations.length-1][0] < target
source: https://leetcode.com/problems/minimum-number-of-refueling-stops/
<div class="separator" style="clear: both;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhaIXRVYnbuUmX14AlXAhpQ1ggfxXb44ufdPDu0lkEcQUL4eoQ2cPrUEnHTKaV9SlrTJO-OADT6fEn0hdYBd5ezvMEYwTb-VQziIOHtZTWGOMtWMpF_vGjAOJhf6z3PnnEM9bs90PO6tYA/s1650/Screen+Shot+2021-06-13+at+12.32.40+PM.png" style="display: block; padding: 1em 0; text-align: center; clear: left; float: left;"><img alt="" border="0" width="320" data-original-height="1438" data-original-width="1650" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhaIXRVYnbuUmX14AlXAhpQ1ggfxXb44ufdPDu0lkEcQUL4eoQ2cPrUEnHTKaV9SlrTJO-OADT6fEn0hdYBd5ezvMEYwTb-VQziIOHtZTWGOMtWMpF_vGjAOJhf6z3PnnEM9bs90PO6tYA/s320/Screen+Shot+2021-06-13+at+12.32.40+PM.png"/></a></div>
Solution:
class Solution {
public int minRefuelStops(int target, int startFuel, int[][] stations) {
PriorityQueue<Integer> pq = new PriorityQueue<>(Collections.reverseOrder());
int count=0;
for(int[] station: stations){
while(startFuel<station[0] && !pq.isEmpty()){
startFuel+=pq.poll();
count++;
}
if(startFuel<station[0])
return -1;
pq.offer(station[1]);
}
while(startFuel<target && !pq.isEmpty()){
startFuel+=pq.poll();
count++;
}
if(startFuel<target)
return -1;
return count;
}
}
Description:
PriorityQueue<Integer> pq = new PriorityQueue<>(Collections.reverseOrder());
Please note Collections.reverseOrder() is passed in constructor as want to build MAX heap.
This heap will store gas capacity in non-decreasing order.
My initial solution(Not Passed):
class Solution {
public int minRefuelStops(int target, int startFuel, int[][] stations) {
PriorityQueue<Integer> pq = new PriorityQueue<>(Collections.reverseOrder());
int count=0;
for(int[] station: stations){
while(startFuel<station[0] && !pq.isEmpty()){
startFuel+=pq.poll();
count++;
}
if(startFuel<0)
return -1;
pq.offer(station[1]);
}
return count;
}
}
This failed following test case:
100
1
[[10,100]]
I realized I made a mistake in the "if" condition. As we keep moving currentLocation needs to checked(why? think) against startFuel.
Now why currentLocation check? See this question has one simplification 1 litre of gas is needed for 1 mile. So gas capacity can just be compared to distance travelled so far. Another simplication is locations can be treated as x coordinate or the distance travel so far. So if you are at station 2 and total distance travelled so far is just location of station 2.
Next solution(Not passed):
class Solution {
public int minRefuelStops(int target, int startFuel, int[][] stations) {
PriorityQueue<Integer> pq = new PriorityQueue<>(Collections.reverseOrder());
int count=0;
for(int[] station: stations){
while(startFuel<station[0] && !pq.isEmpty()){
startFuel+=pq.poll();
count++;
}
if(startFuel<station[0]) // changed this
return -1;
pq.offer(station[1]);
}
return count;
}
}
Failed Test case:
100
10
[[10,60],[20,30],[30,30],[60,40]]
Realised new mistake:
Next attempt(Not passed):
class Solution {
public int minRefuelStops(int target, int startFuel, int[][] stations) {
PriorityQueue<Integer> pq = new PriorityQueue<>(Collections.reverseOrder());
int count=0;
for(int[] station: stations){
while(startFuel<station[0] && !pq.isEmpty()){
startFuel+=pq.poll();
count++;
}
if(startFuel<station[0])
return -1;
pq.offer(station[1]);
}
while(startFuel<target && !pq.isEmpty()){
startFuel+=pq.poll();
count++;
}
return count;
}
}
Failed Test case:
100
1
[]
New mistake uncovered:
Final solution(Passed):
class Solution {
public int minRefuelStops(int target, int startFuel, int[][] stations) {
PriorityQueue<Integer> pq = new PriorityQueue<>(Collections.reverseOrder());
int count=0;
for(int[] station: stations){
while(startFuel<station[0] && !pq.isEmpty()){
startFuel+=pq.poll();
count++;
}
if(startFuel<station[0])
return -1;
pq.offer(station[1]);
}
while(startFuel<target && !pq.isEmpty()){
startFuel+=pq.poll();
count++;
}
if(startFuel<target)
return -1;
return count;
}
}
Bhaarathttp://www.blogger.com/profile/08379295257807934343noreply@blogger.com1tag:blogger.com,1999:blog-9161129402511288001.post-81614740891516076912021-06-05T05:17:00.001-07:002021-06-05T05:22:48.280-07:00Real Estate in India - Will it come down, ever?Real Estate - Always a hot topic. Bhaarathttp://www.blogger.com/profile/08379295257807934343noreply@blogger.com14