Method countOfAtoms

numberOfAtoms.java:2–71

(String formula)

class Solution {
    public String countOfAtoms(String formula) {
        int n = formula.length();
        Stack<HashMap<String,Integer>> stack = new Stack<>();
        stack.push(new HashMap<String,Integer>());
        int index=0;
        // O(N) + O(K)
        //O(N)*(N)
        while(index<n){
            char ch = formula.charAt(index);
            if(ch == '('){
                stack.push(new HashMap<String,Integer>());
                index++;
            }else if(ch == ')'){
                HashMap<String,Integer> curMap = stack.pop();
                index++; //move to the next char
                //find the multiplier
                StringBuilder multiplier = new StringBuilder();
                while(index < n && Character.isDigit(formula.charAt(index))){
                    multiplier.append(formula.charAt(index));
                    index++;
                }
                //multiply the count - ()n
                if(multiplier.length()>0){
                    int m = Integer.parseInt(multiplier.toString());
                    for(String atom : curMap.keySet()){
                        curMap.put(atom, curMap.get(atom) * m);
                    }
                }
                //insert popped map elements into stack top
                for(String atom : curMap.keySet()){
                    stack.peek().put(
                        atom,
                        stack.peek().getOrDefault(atom,0)+curMap.get(atom)
                    );
                }

            }else{ //Atom name and count
                // take the uppercase letter 
                StringBuilder atomName = new StringBuilder();
                atomName.append(ch);
                index++;
                //collect all lowercase letters
                while(index < n && Character.isLowerCase(formula.charAt(index))){
                    atomName.append(formula.charAt(index));
                    index++;
                }
                StringBuilder count = new StringBuilder();
                while(index < n && Character.isDigit(formula.charAt(index))){
                    count.append(formula.charAt(index));
                    index++;
                }
                int c = (count.length()>0)?Integer.parseInt(count.toString()):1;
                stack.peek().put(
                        atomName.toString(),
                        stack.peek().getOrDefault(atomName.toString(),0)+c
                    );
            }
        }
        // 2K