LeetCode 1011. Binary Search, C++ and Python same idea but different outputs - c++

Question

Can anybody take a look at my CPP code?
It's from this problem: https://leetcode.com/problems/capacity-to-ship-packages-within-d-days/
I feel very frustrated cuz though my python code works, my cpp code doesn't work as always. :pensive: Same idea, different results. It drives me crazy.
The correct output should be 15.
My Python code returns the right output 15 while my CPP code returns the wrong output 10.
My python code:
def cnt_days(weights, k):
total, cnt = 0, 1
for w in weights:
if total + w > k:
total = 0
cnt += 1
total += w
print(total, cnt)
return cnt
def shipWithinDays(weights, D):
left = max(weights)
right = max(weights) * len(weights) // D + 1
while left < right:
mid = left + (right - left) // 2
if cnt_days(weights, mid) > D:
left = mid + 1
else:
right = mid
return left
if __name__ == "__main__":
print(shipWithinDays([1,2,3,4,5,6,7,8,9,10], 5))
My cpp code:
#include <iostream>
#include <vector>
#include <algorithm>
using namespace std;
int cnt_days(vector<int>& weights, int K)
{
int total = 0, cnt = 1;
for (int w: weights)
{
if (total + w > K)
{
total = 0;
cnt++;
}
else total += w;
}
cout << total <<" "<< cnt << endl;
return cnt;
}
int shipWithinDays(vector<int>& weights, int D)
{
int maximum = *max_element(weights.begin(), weights.end());
int left = maximum;
int right = maximum * weights.size() / D + 1;
while (left < right)
{
int mid = left + (right - left)/2;
if (cnt_days(weights, mid) > D)
left = mid + 1;
else
right = mid;
}
return left;
}
int main()
{
vector<int> weights = {1,2,3,4,5,6,7,8,9,10};
int D = 5;
cout << shipWithinDays(weights, D) << endl;
return 0;
}

Answer 1

Maybe, you could binary search much simple for your c++ solution:
#include <vector>
class Solution {
public:
int shipWithinDays(std::vector<int>& weights, int d) {
int lo = 0;
int hi = INT_MAX;
for (int weight : weights) {
lo = max(lo, weight);
}
while (lo < hi) {
int mid = lo + (hi - lo) / 2;
int required = 1;
int cur = 0;
for (int index = 0; index < weights.size() && required <= d; cur += weights[index++])
if (cur + weights[index] > mid) {
cur = 0;
required++;
}
if (required > d) {
lo = mid + 1;
} else {
hi = mid;
}
}
return lo;
}
};
Or with basic bitwise operations:
class Solution {
public:
int shipWithinDays(vector<int> &weights, int d) {
int lo = 0;
int hi = INT_MAX;
for (int weight : weights)
lo = max(lo, weight);
while (lo < hi) {
int mid = lo + ((hi - lo) >> 1);
int required = 1;
int cur = 0;
for (int index = 0; index < weights.size() && required <= d; cur += weights[index++])
if (cur + weights[index] > mid)
cur = 0, required++;
if (required > d)
lo = -~mid;
else
hi = mid;
}
return lo;
}
};
Similarly for python:
from typing import List
class Solution:
def shipWithinDays(self, weights: List[int], d: int) -> int:
lo, hi = max(weights), sum(weights)
while lo < hi:
mid = lo + ((hi - lo) >> 1) # or mid = (lo + hi) // 2 || mid = lo + (hi - lo) // 2
cur, required = 0, 1
for weight in weights:
if cur + weight > mid:
required += 1
cur = 0
cur += weight
if required > d:
lo = -~mid # simply lo = mid + 1
else:
hi = mid
return lo
References
For additional details, you can see the Discussion Board. There are plenty of accepted solutions, explanations, efficient algorithms with a variety of languages, and time/space complexity analysis in there.

Answer 2

remove the else
else total += w;
should be
total += w;