241. Different Ways to Add Parentheses

package LeetCode_241

import java.util.*
import kotlin.collections.ArrayList

/**
 * 241. Different Ways to Add Parentheses
 * https://leetcode.com/problems/different-ways-to-add-parentheses/description/
 * Given a string of numbers and operators, return all possible results from computing all the different possible ways to group numbers and operators.
 * The valid operators are +, - and *.
 *
Example 1:
Input: "2-1-1"
Output: [0, 2]
Explanation:
((2-1)-1) = 0
(2-(1-1)) = 2

Example 2:
Input: expression = "2*3-4*5"
Output: [-34,-14,-10,-10,10]
Explanation:
(2*(3-(4*5))) = -34
((2*3)-(4*5)) = -14
((2*(3-4))*5) = -10
(2*((3-4)*5)) = -10
(((2*3)-4)*5) = 10

Constraints:
1. 1 <= expression.length <= 20
2. expression consists of digits and the operator '+', '-', and '*'.
 * */
class Solution {
    /*
    * solution:DFS, Memorization with map, split each char to check it digit or symbol and calculate value with operations from left to right
    * Time:O(n*2^n), Space:O(2^n)
    * */
    fun diffWaysToCompute(expression: String): List<Int> {
        val map = HashMap<String, List<Int>>()
        return dfs(expression, map)
    }

    private fun dfs(expression: String, map: HashMap<String, List<Int>>): List<Int> {
        if (map.containsKey(expression)){
            return map.getOrDefault(expression, ArrayList<Int>())
        }
        val values = ArrayList<Int>()
        if (!hasOperations(expression)) {
            //just put digit in values
            values.add(expression.toInt())
        } else {
            for (i in expression.indices) {
                val c = expression[i]
                if (!c.isDigit()) {
                    val left = dfs(expression.substring(0, i), map)
                    //substring form current i to the end
                    val right = dfs(expression.substring(i + 1), map)
                    //check each digit from left sub list and right sub list
                    for (l in left) {
                        for (r in right) {
                            when (c) {
                                '+' -> values.add(l + r)
                                '-' -> values.add(l - r)
                                '*' -> values.add(l * r)
                            }
                        }
                    }
                }
            }
        }
        //save into map for next level
        map.put(expression, values)
        return values
    }

    private fun hasOperations(expression: String):Boolean{
        for (c in expression){
            if (c=='+' || c=='-' || c=='*'){
                return true
            }
        }
        return false
    }
}