399. Evaluate Division

Posted 2020-12-13 wentiliangkaihua

Equations are given in the format A / B = k, where A and B are variables represented as strings, and k is a real number (floating point number). Given some queries, return the answers. If the answer does not exist, return -1.0.

Example:
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 ].

The input is: vector<pair<string, string>> equations, vector<double>& values, vector<pair<string, string>> queries , where equations.size() == values.size(), and the values are positive. This represents the equations. Return vector<double>.

According to the example above:

equations = [ ["a", "b"], ["b", "c"] ],
values = [2.0, 3.0],
queries = [ ["a", "c"], ["b", "a"], ["a", "e"], ["a", "a"], ["x", "x"] ]. 

class Solution {
    Map<String, Map<String, Double>> g = new HashMap();
    public double[] calcEquation(List<List<String>> equations, double[] values, List<List<String>> queries) {
        
        for(int i = 0; i < equations.size(); i++){
            String fir = equations.get(i).get(0);
            String sec = equations.get(i).get(1);
            double val = values[i];            
            g.putIfAbsent(fir, new HashMap());
            g.putIfAbsent(sec, new HashMap());
            g.get(fir).putIfAbsent(sec, val);
            g.get(sec).putIfAbsent(fir, 1.0/val);
            
            //map.get(fir).put(sec, val);
        }
        double[] ans = new double[queries.size()];
    
        for (int i = 0; i < queries.size(); ++i) {      
          String x = queries.get(i).get(0);
          String y = queries.get(i).get(1);
          if (!g.containsKey(x) || !g.containsKey(y)) {
            ans[i] = -1.0;
          } else {        
            ans[i] = divide(x, y, new HashSet<String>());
          }
        }

        return ans;
        //return new double[]{};
    }
    
     private double divide(String x, String y, Set<String> visited) {
        if (x.equals(y)) return 1.0;
        visited.add(x);
        if (!g.containsKey(x)) return -1.0;
        for (String n : g.get(x).keySet()) {
          if (visited.contains(n)) continue;
          visited.add(n);
          double d = divide(n, y, visited);
          if (d > 0) return d * g.get(x).get(n);
        }
        return -1.0;
    }
}

1. 阳间方法，graph + dfs

http://zxi.mytechroad.com/blog/graph/leetcode-399-evaluate-division/花哥讲解

看着代码长其实挺好理解，先用Map<String, Map<String, Double>>来表示有向图，注意权重值不同，每条路径唯一

然后iterate queries，如果graph中没有这个key说明要返回-1，否则进入dfs中，

dfs参数是分子，分母，visited（确保不进入死循环）

如果这一步了找不到分子了就直接返回-1，找到x的话遍历它的邻居们n，如果邻居能牵线搭桥到y，说明此路通，否则此路不通

 

 2. 阴间方法，union find

挖个坑