1578. Minimum Time to Make Rope Colorful
Approach
Greedy — for each run of same color balloons keep the max-cost one and sum the rest.
Key Techniques
Array problems involve manipulating elements stored in a contiguous block of memory. Key techniques include two-pointer traversal, prefix sums, sliding windows, and in-place partitioning. In C#, arrays are zero-indexed and fixed in size — use List<T> when you need dynamic resizing.
String problems range from simple character counting to complex pattern matching. Common approaches include two pointers, sliding window, prefix hashing, and the KMP algorithm. In C#, strings are immutable — use StringBuilder for efficient concatenation inside loops.
Dynamic programming solves problems by breaking them into overlapping sub-problems and storing results to avoid redundant work. The key steps are: define state, write a recurrence relation, set base cases, and choose top-down (memoization) or bottom-up (tabulation). DP often yields O(n²) → O(n) time improvements over brute force.
// Approach: Greedy — for each run of same color balloons keep the max-cost one and sum the rest.
// Time: O(n) Space: O(1)
public class Solution
{
public int MinCost(string colors, int[] neededTime)
{
int n = colors.Length;
int ans = 0;
int maxNeededTime = neededTime[0];
for (int i = 1; i < n; i++)
{
if (colors[i - 1] == colors[i])
{
ans = ans + Math.Min(maxNeededTime, neededTime[i]);
maxNeededTime = Math.Max(maxNeededTime, neededTime[i]);
}
else
maxNeededTime = neededTime[i];
}
return ans;
}
}