# 726. Number of Atoms

```java
class Solution {
    public String countOfAtoms(String formula) {
        StringBuilder sb = new StringBuilder();
        Map<String, Integer> memo= helper(formula, 0).getKey();

        List<String> atoms= new ArrayList<>(memo.keySet());
        Collections.sort(atoms);
        
        for(String str: atoms){
            sb.append(str+ (memo.get(str)==1? "" : memo.get(str)));
        }
        return sb.toString();
    }
    
    private Pair<Map<String, Integer>, Integer> helper(String formula, int start){
        Map<String, Integer> memo = new HashMap<>();
        int index = start;
        while(index<formula.length()){
            if(formula.charAt(index)>='A' && formula.charAt(index)<='Z'){
                index = countKey(formula, memo, index);
            }else if(formula.charAt(index)=='('){
                Pair<Map<String, Integer>, Integer> pair = helper(formula, index+1);
                Map<String, Integer> subMemo = pair.getKey();
                // add subMemo to memo
                for(String key : subMemo.keySet()){
                    memo.put(key, memo.getOrDefault(key, 0)+subMemo.get(key));
                }
                index = pair.getValue();
            }else if(formula.charAt(index) == ')'){
                // get the count after ')'
                int count=1;
                int subStart = ++index;
                while(index<formula.length() && formula.charAt(index)>='0' && formula.charAt(index)<='9'){
                    index++;
                }
                if(index>subStart){
                    count=Integer.parseInt(formula.substring(subStart, index));
                }
                for(String key : memo.keySet()){
                    memo.put(key, memo.get(key)*count);
                }

                return new Pair(memo, index);
            }
        }
        return new Pair(memo, index);
    }
    
    private int countKey(String formular, Map<String, Integer> memo, int start){
        // start will always be the upper case letter
        int end = start+1;
        while(end<formular.length() && formular.charAt(end)>='a' && formular.charAt(end)<='z'){
            end++;
        }
        String key = formular.substring(start, end);
        
        // now get the integer after the key
        int count = 1;
        int subStart = end;
        while(end <formular.length() && formular.charAt(end)>='0' && formular.charAt(end)<='9'){
            end++;
        }
        if (end>subStart){
            count= Integer.parseInt(formular.substring(subStart, end));
        }
        memo.put(key, memo.getOrDefault(key, 0)+count);
        return end;
    }
}
```


---

# Agent Instructions: Querying This Documentation

If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter:

```
GET https://ucan.gitbook.io/notes/stack/726.-number-of-atoms.md?ask=<question>
```

The question should be specific, self-contained, and written in natural language.
The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.