Create 862. Shortest Subarray with Sum at Least K

862. Shortest Subarray with Sum at Least K

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+class Solution { 
+public:
+    int shortestSubarray(vector<int>& nums, int k) {
+        int n=nums.size();
+
+        // find the prefix sum of the given array nums -: 
+        // Size is n+1 to handle subarrays starting from index 0
+        vector<long long>prefixSum(n+1,0); 
+
+        // Calculating prefix sums
+        for(int i=1; i<=n; i++){
+            prefixSum[i]=prefixSum[i-1]+nums[i-1];
+        }
+
+        // Make a deque (doubly-ended queue -: which allows us to add/remove ele.s
+        // from both front/end side. 
+        deque<int>candiIdx;
+
+        int len=INT_MAX;
+
+        for(int i=0; i<=n; i++){
+            // Remove candidates from front of deque where subarray sum meets target sum
+            while(!candiIdx.empty() && prefixSum[i]-prefixSum[candiIdx.front()]>=k){
+                len=min(len,i-candiIdx.front());
+                candiIdx.pop_front();
+            }
+            // Maintain monotonicity by removing indices with larger prefix sums
+            while(!candiIdx.empty() && prefixSum[i]<=prefixSum[candiIdx.back()]) {
+                candiIdx.pop_back();
+            }
+            // Add current index to candidates
+            candiIdx.push_back(i);
+        }
+        // Return -1 if no valid subarray found
+        return len == INT_MAX ? -1 : len;
+   }
+};