1488. Avoid Flood in The City (#901)

Author: Chayandas07

1488. Avoid Flood in The City

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+class Solution {
+public:
+    vector<int> avoidFlood(vector<int>& rains) {
+        set<int> zer; // stores indices of days we can dry (rains[i] == 0)
+        int n = rains.size();
+        vector<int> ans(n, -1); // result array
+        unordered_map<int, int> M; // lake -> last day it was filled
+
+        for (int i = 0; i < n; i++) {
+            if (rains[i] == 0) {
+                // We can dry some lake later
+                zer.insert(i);
+                ans[i] = 1; // default, can be updated later
+            } else {
+                int lake = rains[i];
+                // If lake already has water, we need to dry it before raining again
+                if (M.find(lake) != M.end()) {
+                    int lastRainDay = M[lake];
+                    // Find first zero-day after last rain day
+                    auto up = zer.upper_bound(lastRainDay);
+                    if (up == zer.end()) {
+                        // No zero-day available to dry => flood
+                        return {};
+                    }
+                    // Use that day to dry the lake
+                    ans[*up] = lake;
+                    zer.erase(up);
+                }
+                // Update the last rain day for this lake
+                M[lake] = i;
+            }
+        }
+        return ans;
+    }
+};