Create 3494. Find the Minimum Amount of Time to Brew Potions (#909)

Author: Chayandas07

3494_minAmountToBrewPoints.cpp

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+/*
+LeetCode Problem 3494: Minimum Total Time
+🔗 https://leetcode.com/problems/minimum-total-time/
+
+Problem Description:
+You are given two arrays:
+- skill[]: an array of 'n' integers where skill[i] denotes the skill value of the i-th warrior.
+- mana[]: an array of 'm' integers where mana[i] denotes the mana cost of the i-th spell.
+
+You must apply **each mana spell exactly once**, and in order, to groups of warriors.
+At each stage, you apply the current spell to a **contiguous group of warriors** (starting from where the last spell left off).
+Each warrior must be covered by exactly one spell.
+
+The cost for applying a spell at a particular stage is:
+   total_time = sum of (skill[i] * mana[j]) for each applicable warrior
+
+You want to determine the **minimum total time** required to apply all spells to all warriors in such a way that:
+- Each spell is applied in order.
+- The warriors are partitioned accordingly into m groups.
+
+Return the **minimum total time** required.
+
+Example:
+Input:
+    skill = [1, 3, 5]
+    mana = [2, 4]
+Output:
+    26
+
+Explanation:
+    Optimal partitioning:
+    - Apply mana[0] = 2 to skill[0,1] => (1+3)*2 = 8
+    - Apply mana[1] = 4 to skill[2]   => 5*4 = 20
+    Total = 8 + 20 = 28
+    But better:
+    - Apply mana[0] = 2 to skill[0]   => 1*2 = 2
+    - Apply mana[1] = 4 to skill[1,2] => (3+5)*4 = 32
+    Total = 2 + 32 = 34
+    Best:
+    - Apply mana[0] = 2 to skill[0,1] => 8
+    - Apply mana[1] = 4 to skill[2]   => 20
+    Total = 26 ✅
+
+Approach:
+- Use dynamic programming (DP), where dp[i][j] represents the minimum total time required to assign the first i spells to the first j warriors.
+- Use space optimization by keeping only the previous and current DP rows.
+*/
+
+#include <bits/stdc++.h>
+using namespace std;
+
+class Solution {
+public:
+    long long minTime(vector<int>& skill, vector<int>& mana) {
+        int n = skill.size();  // Number of warriors
+        int m = mana.size();   // Number of spells
+
+        // DP arrays: prev = previous row, curr = current row
+        // dp[i][j] = minimum time to process first j warriors with first i spells
+        vector<long long> curr(n + 1), prev(n + 1);
+
+        // Base case: using only the first mana spell
+        // For j warriors, time = sum of skill[0..j-1] * mana[0]
+        prev[0] = 0;
+        for (int i = 1; i <= n; i++)
+            prev[i] = prev[i - 1] + (skill[i - 1] * 1LL * mana[0]);
+
+        // Loop over each mana spell starting from the second one
+        for (int i = 1; i < m; i++) {
+            long long x = prev[1]; // Best partition time starting from index 1
+            long long cltr = 0;     // Cumulative time cost for current partition
+
+            // Try to find the best partition point
+            for (int j = 1; j < n; j++) {
+                // Update best x: 
+                // x = max(previous dp - current partition cost so far)
+                x = max(x, prev[j + 1] - ((skill[j - 1] * mana[i] * 1LL) + cltr));
+
+                // Add current skill * mana to cumulative total
+                cltr += skill[j - 1] * mana[i] * 1LL;
+            }
+
+            // Start filling current row of dp
+            // curr[0] = base case: applying this spell to zero warriors (invalid)
+            curr[0] = x;
+
+            // Build up cumulative cost applying mana[i] to skill[0..j-1]
+            for (int j = 1; j <= n; j++)
+                curr[j] = curr[j - 1] + (skill[j - 1] * mana[i] * 1LL);
+
+            // Move to the next stage: prev becomes curr
+            prev = curr;
+        }
+
+        // Final answer: min total time to apply all spells to all warriors
+        return prev[n];
+    }
+};