Evaluate Division

Published: Oct 20, 2022

Introduction

This problem might look like a simple math, and a greedy approach works. However, creating a graph works better. The path, a to b’s edge has a weight v, while b to a’s edge has a weight 1 / v. While visiting edges, it’s weight will be multiplied. The graph traversal can do by either of BFS or DFS.

Problem Description

You are given an array of variable pairs equations and an array of real numbers values, where equations[i] = [A[i], B[i]] and values[i] represent the equation A[i] / B[i] = values[i]. Each A[i] or B[i] is a string that represents a single variable.

You are also given some queries, where queries[j] = [C[j], D[j]] represents the j-th query where you must find the answer for C[j] / D[j] = ?.

Return the answers to all queries. If a single answer cannot be determined, return -1.0.

Note: The input is always valid. You may assume that evaluating the queries will not result in division by zero and that there is no contradiction.

Constraints:

• 1 <= equations.length <= 20
• equations[i].length == 2
• 1 <= A[i].length, Bi.length <= 5
• values.length == equations.length
• 0.0 < values[i] <= 20.0
• 1 <= queries.length <= 20
• queries[i].length == 2
• 1 <= C[j].length, D[j].length <= 5
• A[i], B[i], C[j], D[j] consist of lower case English letters and digits.

https://leetcode.com/problems/evaluate-division/

Examples

Example 1
Input: equations = [["a","b"],["b","c"]], values = [2.0,3.0], queries = [["a","c"],["b","a"],["a","e"],["a","a"],["x","x"]]
Output: [6.00000,0.50000,-1.00000,1.00000,-1.00000]
Explanation: 
Given: a / b = 2.0, b / c = 3.0
queries are: a / c = ?, b / a = ?, a / e = ?, a / a = ?, x / x = ?
return: [6.0, 0.5, -1.0, 1.0, -1.0 ]

Example 2
Input: equations = [["a","b"],["b","c"],["bc","cd"]], values = [1.5,2.5,5.0], queries = [["a","c"],["c","b"],["bc","cd"],["cd","bc"]]
Output: [3.75000,0.40000,5.00000,0.20000]

Example 3
Input: equations = [["a","b"]], values = [0.5], queries = [["a","b"],["b","a"],["a","c"],["x","y"]]
Output: [0.50000,2.00000,-1.00000,-1.00000]

Analysis

The first step is to build a graph. The adjacency list has a neighbor of itself to handle the [“a”, “a”] query. The neighbor list is a tuple of node and edge value.

The second step is a search. Except edge value calculation, it is nothing special. When a popped out node is the destination, the path is found along with the edge value. If not, add the neighbor node with updated edge value. If queue becomes empty without finding the destination, the path doesn’t exist.

Solution

class EvaluateDivision:
    def calcEquation(self, equations: List[List[str]], values: List[float], queries: List[List[str]]) -> List[float]:
        graph = {}
        
        def buildGraph():
            for i in range(len(equations)):
                s, d, v = equations[i][0], equations[i][1], values[i]
                if s not in graph:
                    graph[s] = [(s, 1)]
                if d not in graph:
                    graph[d] = [(d, 1)]
                graph[s].append((d, v))
                graph[d].append((s, 1 / v))

        def search(src, dst):
            if src not in graph or dst not in graph:
                return -1
            queue = [(src, 1)]
            seen = {src}
            while queue:
                cur_n, cur_v = queue.pop(0)
                for next_n, next_v in graph[cur_n]:
                    if next_n == dst:
                        return cur_v * next_v
                    elif next_n not in seen:
                        seen.add(next_n)
                        queue.append((next_n, cur_v * next_v))
            return -1
        
        buildGraph()
        return [search(s, d) for s, d in queries]

Complexities

• Time: O(mn) – m: number of queries, n: number of equations
• Space: O(n)