5 Must Solve LeetCode Problems

If I were given the option to solve the 5 most important leetcode problems I would solve these 5 problems. I am sharing the list of 5 problems with a solution.

1. Valid Parenthesis

class Solution {
    public boolean isValid(String s) {
        Stack<Character> stack = new Stack<Character>();
       for (char c : s.toCharArray()) { 
            if (c == '(' || c=='{' || c=='[' ) 
                stack.push(c); 
            
            else if(c==')'){
                if(!stack.empty() && stack.peek()=='(') stack.pop();
                else return false;
            } 

            else if (c=='}'){
                if(!stack.empty() && stack.peek()=='{') stack.pop();
                else return false;
            } 
             else if (c==']'){
                if(!stack.empty() && stack.peek()=='[') stack.pop();
                else return false;
            } 
        }
        if(stack.empty()) return true;
        return false;
   
    }
}

2. Best Time to buy and sell stock

public int maxProfit(int[] prices) {

        // int profit = 0;
        // int buyAt = Integer.MAX_VALUE;
        // for (int price : prices) {
        //     if (buyAt < price) {
        //         profit += price - buyAt;
        //     }
        //     buyAt = price;
        // }
        // return profit;

        int profit = 0;
        for (int i = 1;i<prices.length;i++) {
          if (prices[i] > prices[i-1]) {
              profit += prices[i] - prices[i-1];
          }
        }
        return profit;
    }

3. Combination Sum

public List<List<Integer>> combinationSum(int[] candidates, int target) {
        List<List<Integer>> list= new ArrayList<List<Integer>>();
        find(list,candidates,target,new ArrayList<Integer>(),0);
        return list;
    }
    
    
    private void find(List<List<Integer>> list,int[] candidates,int target,ArrayList<Integer> temp,int i)
    {
        if(target==0)
        {
            list.add(new ArrayList<Integer>(temp));
            return;
        }
        if(i==candidates.length)
           return;
        
        // pick
        if(candidates[i]<=target){
            temp.add(candidates[i]);
            find(list,candidates,target-candidates[i],temp,i);
            temp.remove(temp.size()-1);
        }

        // not pick
        find(list,candidates,target,temp,i+1);
    }

4. Two Sum

public int[] twoSum(int[] nums, int target) {
        
        Map<Integer, Integer> map = new HashMap<>();
        int n = nums.length;
        int[] res = new int[2];

        for (int i = 0;i<n;i++) {
            if (map.containsKey(target - nums[i])) {
                res[0] = map.get(target - nums[i]);
                res[1] = i;
            } else {
                map.put(nums[i], i);
            }
        }
        return res;
    }

5. Basic Calculator

public int calculate(String s) {
        //Use stack for evaluating expression
        int len = s.length();
        if(s==null || (len = s.length())==0) return 0;

        Stack<Integer> stack = new Stack<>();
        int num = 0;
        char sign = '+';

        for (int i = 0;i<len;i++) {
            if (Character.isDigit(s.charAt(i))) {
                num = num*10 + s.charAt(i) - '0';
            }
            if((!Character.isDigit(s.charAt(i)) &&' '!=s.charAt(i)) || i==len-1){
            if(sign=='-'){
                stack.push(-num);
            }
            if(sign=='+'){
                stack.push(num);
            }
            if(sign=='*'){
                stack.push(stack.pop()*num);
            }
            if(sign=='/'){
                stack.push(stack.pop()/num);
            }
            sign = s.charAt(i);
            num = 0;
          }
        }
        int re = 0;
        for(int i:stack){
            re += i;
        }
        return re;
    }

So above are five commonly asked leet code problems of tech interviews, I will be sharing more in the subsequent articles.

Thanks for reading!!