Leetcode 726 study Java code first, and then write a C# solution.

Leetcode726_studyCode.java

// source code from http://zxi.mytechroad.com/blog/string/leetcode-726-number-of-atoms/
// Author: Huahua
// Runtime: 13 ms
// Feb. 1, 2018 plan to study the code and write a C# solution 
class Solution {
    private int i;    
    public String countOfAtoms(String formula) {
        StringBuilder ans = new StringBuilder();
        i = 0;
        Map<String, Integer> counts = countOfAtoms(formula.toCharArray());
        for (String name: counts.keySet()) {
            ans.append(name);
            int count = counts.get(name);
            if (count > 1) ans.append("" + count);
        }
        return ans.toString();
    }
    
    private Map<String, Integer> countOfAtoms(char[] f) {
        Map<String, Integer> ans = new TreeMap<String, Integer>();
        while (i != f.length) {
            if (f[i] == '(') {
                ++i;
                Map<String, Integer> tmp = countOfAtoms(f);
                int count = getCount(f);
                for (Map.Entry<String, Integer> entry : tmp.entrySet())
                    ans.put(entry.getKey(), 
                            ans.getOrDefault(entry.getKey(), 0) 
                            + entry.getValue() * count);
            } else if (f[i] == ')') {
                ++i;
                return ans;
            } else {
                String name = getName(f);
                ans.put(name, ans.getOrDefault(name, 0) + getCount(f));
            }
        }
        return ans;
    }
    
    private String getName(char[] f) {
        String name = "" + f[i++];
        while (i < f.length && 'a' <= f[i] && f[i] <= 'z') name += f[i++];
        return name;
    }
    
    private int getCount(char[] f) {
        int count = 0;
        while (i < f.length && '0' <= f[i] && f[i] <= '9') {
            count = count * 10 + (f[i] - '0');
            ++i;
        }
        return count == 0 ? 1 : count;
    }
}