# 0726. Number of Atoms

https://leetcode.com/problems/number-of-atoms

## Description

Given a string `formula` representing a chemical formula, return _the count of each atom_.

The atomic element always starts with an uppercase character, then zero or more lowercase letters, representing the name.

One or more digits representing that element's count may follow if the count is greater than `1`. If the count is `1`, no digits will follow.

* For example, `"H2O"` and `"H2O2"` are possible, but `"H1O2"` is impossible.

Two formulas are concatenated together to produce another formula.

* For example, `"H2O2He3Mg4"` is also a formula.

A formula placed in parentheses, and a count (optionally added) is also a formula.

* For example, `"(H2O2)"` and `"(H2O2)3"` are formulas.

Return the count of all elements as a string in the following form: the first name (in sorted order), followed by its count (if that count is more than `1`), followed by the second name (in sorted order), followed by its count (if that count is more than `1`), and so on.

**Example 1:**

```
**Input:** formula = "H2O"
**Output:** "H2O"
**Explanation:** The count of elements are {'H': 2, 'O': 1}.
```

**Example 2:**

```
**Input:** formula = "Mg(OH)2"
**Output:** "H2MgO2"
**Explanation:** The count of elements are {'H': 2, 'Mg': 1, 'O': 2}.
```

**Example 3:**

```
**Input:** formula = "K4(ON(SO3)2)2"
**Output:** "K4N2O14S4"
**Explanation:** The count of elements are {'K': 4, 'N': 2, 'O': 14, 'S': 4}.
```

**Example 4:**

```
**Input:** formula = "Be32"
**Output:** "Be32"
```

**Constraints:**

* `1 <= formula.length <= 1000`
* `formula` consists of English letters, digits, `'('`, and `')'`.
* `formula` is always valid.
* All the values in the output will fit in a 32-bit integer.

## ac

similar: [https://leetcode.com/problems/decode-string](https://leetcode.com/problems/decode-string)

```java
class Solution {
    int i = 0;
    public String countOfAtoms(String formula) {
        // edge cases
        if (formula == null || formula.length() == 0) return "";

        Map<String, Integer> res = count(formula);

        List<String> list = new ArrayList<>();
        list.addAll(res.keySet());
        Collections.sort(list);

        StringBuilder sb = new StringBuilder();
        for (String s : list) {
            sb.append(s);
            int val = res.get(s);
            if (val > 1) sb.append(val);
        }

        return sb.toString();
    }

    //formula = "H2O"
    private Map<String, Integer> count(String s) {
        Map<String, Integer> res = new HashMap<>();

        while (i < s.length()) {
            if (Character.isUpperCase(s.charAt(i))) {
                // get upper case
                StringBuilder sb = new StringBuilder();
                sb.append(s.charAt(i));
                i++;
                // get lower case
                while (i < s.length() && Character.isLowerCase(s.charAt(i))) {
                    sb.append(s.charAt(i));
                    i++;
                }
                String atomic = sb.toString();
                // get count
                int n = 0;
                while (i < s.length() && Character.isDigit(s.charAt(i))) {
                    n = n * 10 + (s.charAt(i) - '0');
                    i++;
                }
                n = n > 0 ? n : 1;
                res.put(atomic, res.getOrDefault(atomic, 0) + n); // update
            } else if (s.charAt(i) == '(') {
                i++;
                Map<String, Integer> child = count(s);
                // add child to current map
                for (String key : child.keySet()) {
                    res.put(key, child.get(key) + res.getOrDefault(key, 0));
                }
            } else if (s.charAt(i) == ')') { // exit this recursion
                i++;
                // count after parenthesis
                int n = 0;
                while (i < s.length() && Character.isDigit(s.charAt(i))) {
                    n = n * 10 + (s.charAt(i) - '0');
                    i++;
                }
                if (n > 0) { // update count
                    for (Map.Entry<String, Integer> entry : res.entrySet()) {
                        entry.setValue(n * entry.getValue());
                    }
                }
                return res;
            }
        }

        return res;
    }
}
```
