Create 3169. Count Days Without Meetings (#750)

Author: Chayandas07

3169. Count Days Without Meetings

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+class Solution {
+public:
+    int countDays(int days, vector<vector<int>>& meetings) {
+        const int n = meetings.size(), N=2*n;
+        vector<unsigned> info(N);
+        int i=0;
+        
+        // Encode meeting start and end 
+        for (auto& m : meetings) {
+            const unsigned s= m[0], e=m[1];
+            info[i++]=(s<<1)|1;  // Start, mark with LSB=1
+            info[i++]=(e+1)<<1;  // End (exclusive)
+        }
+
+        // Sort the events
+        sort(info.begin(), info.end());
+
+        int overlap=0, cnt=0, last=1;
+        
+        // Process events in sorted order
+        for (int i=0; i<N; i++) {
+            const int x=info[i]>>1;    // Extract day
+            const bool isStart=info[i]&1;
+
+            // If no overlap, count the days between last and current 
+            if (overlap==0 && last<x) 
+                cnt+=(x-last);
+
+            overlap+=isStart?1:-1;  // Increment on start, decrement on end
+            if(overlap==0) last=x;   // Update last when no overlap
+        }
+
+        // free between last & days
+        if (last<=days) 
+            cnt+=(days-last+1);
+
+        return cnt;
+    }
+};