Create 726. Number of Atoms (#529)

Author: Chayandas07

726. Number of Atoms

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+
+
+class Solution {
+public:
+    string countOfAtoms(string formula) {
+        stack<map<string, int>> stk;
+        stk.push(map<string, int>());
+        int n = formula.size();
+        for (int i = 0; i < n;) {
+            if (formula[i] == '(') {
+                stk.push(map<string, int>());
+                i++;
+            } else if (formula[i] == ')') {
+                auto top = stk.top();
+                stk.pop();
+                i++;
+                int i_start = i;
+                while (i < n && isdigit(formula[i])) {
+                    i++;
+                }
+                int multiplier = i > i_start ? stoi(formula.substr(i_start, i - i_start)) : 1;
+                for (auto &p : top) {
+                    stk.top()[p.first] += p.second * multiplier;
+                }
+            } else {
+                int i_start = i;
+                i++;
+                while (i < n && islower(formula[i])) {
+                    i++;
+                }
+                string element = formula.substr(i_start, i - i_start);
+                i_start = i;
+                while (i < n && isdigit(formula[i])) {
+                    i++;
+                }
+                int count = i > i_start ? stoi(formula.substr(i_start, i - i_start)) : 1;
+                stk.top()[element] += count;
+            }
+        }
+        auto &counts = stk.top();
+        vector<pair<string, int>> elements(counts.begin(), counts.end());
+        sort(elements.begin(), elements.end());
+        string result;
+        for (auto &p : elements) {
+            result += p.first;
+            if (p.second > 1) {
+                result += to_string(p.second);
+            }
+        }
+        return result;
+    }
+};