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# Calculate Entropy LeetCode Solution

## Problem – Calculate Entropy LeetCode Solution

Given a group of values, the entropy of the group is defined as the formula as following:

where `P(x)` is the probability of appearance for the value `x`.

The exercise is to calculate the entropy of a group. Here is one example.

the input group:  `[1, 1, 2, 2]`

the probability of value 1 is  2/4 = 1/2
the probability of value 2 is  2/4 = 1/2

As a result, its entropy can be obtained by:  `- (1/2) * log2(1/2) - (1/2) * log2(1/2) = 1/2 + 1/2 = 1`

Note: the precision of result would remain within 1e-6.

### Calculate Entropy LeetCode Solution in C++

``````    for (const pair<int,int>& p : um) {
const int count = p.second;
double logProbability = log2(static_cast<double>(count)/n);

ans -= (static_cast<double>(count)/n) * log2(static_cast<double>(count)/n);
}

return ans;
}
``````

### Calculate Entropy LeetCode Solution in Java

``````class Result {
Map<List<String>, Integer> map;

public Result() {
map = new HashMap<>();
}

public Result(Map<List<String>, Integer> map) {
this.map = map;
}

public void update(List<String> key, int val) {
map.put(key, map.getOrDefault(key, 0) + val);
}

public Map<List<String>, Integer> getMap() {
return map;
}

public List<String> toList() {
List<List<String>> keyList = new ArrayList<>(map.keySet());
Map<List<String>, String> list2String = new HashMap();
for (List<String> key : keyList) {
StringBuilder sb = new StringBuilder();
for (String k : key) {
sb.append(k + "*");
}
list2String.put(key, sb.toString());
}

Collections.sort(keyList, (a, b) -> (a.size() == b.size() ? list2String.get(a).compareTo(list2String.get(b)) : b.size() - a.size()));

List<String> res = new ArrayList<>();
for (List<String> key : keyList) {
if (map.get(key) == 0) continue;
StringBuilder sb = new StringBuilder();
sb.append(map.get(key));
for (String k : key) {
sb.append("*" + k);
}
}
return res;
}

}

Map<String, Integer> evalMap;
int i = 0;
public List<String> basicCalculatorIV(String expression, String[] evalvars, int[] evalints) {
evalMap = new HashMap<>();
for (int j = 0; j < evalvars.length; j++) {
evalMap.put(evalvars[j], evalints[j]);
}
i = -1;
next(expression);
Result res = expression(expression);
return res.toList();
}

private Result expression(String s) {
Result res = term(s);
while (i < s.length() && (s.charAt(i) == '+' || s.charAt(i) == '-')) {
int c = s.charAt(i);
next(s);
if (c == '+') {
} else {
res = substract(res, term(s));
}
}
return res;
}

private Result term(String s) {
Result res = factor(s);
while (i < s.length() && s.charAt(i) == '*') {
int c = s.charAt(i);
next(s);
res = multiply(res, factor(s));
}
return res;
}

public Result multiply(Result r1, Result r2) {
Map<List<String>, Integer> map1 = r1.getMap();
Map<List<String>, Integer> map2 = r2.getMap();
Map<List<String>, Integer> map = new HashMap<>();
for (List<String> key1 : map1.keySet()) {
for (List<String> key2 : map2.keySet()) {
List<String> key = new ArrayList<>(key1);
Collections.sort(key);
map.put(key, map.getOrDefault(key, 0) + map1.get(key1) * map2.get(key2));
}
}
return new Result(map);
}

public Result add(Result r1, Result r2) {
Map<List<String>, Integer> map1 = r1.getMap();
Map<List<String>, Integer> map2 = r2.getMap();
Map<List<String>, Integer> map = new HashMap<>();
for (List<String> key1 : map1.keySet()) {
map.put(key1, map.getOrDefault(key1, 0) + map1.get(key1));
}
for (List<String> key2 : map2.keySet()) {
map.put(key2, map.getOrDefault(key2, 0) + map2.get(key2));
}
return new Result(map);
}

public Result substract(Result r1, Result r2) {
Map<List<String>, Integer> map1 = r1.getMap();
Map<List<String>, Integer> map2 = r2.getMap();
Map<List<String>, Integer> map = new HashMap<>();
for (List<String> key1 : map1.keySet()) {
map.put(key1, map.getOrDefault(key1, 0) + map1.get(key1));
}
for (List<String> key2 : map2.keySet()) {
map.put(key2, map.getOrDefault(key2, 0) - map2.get(key2));
}
return new Result(map);
}

private Result factor(String s) {
Result res = new Result();
if (s.charAt(i) == '(') {
next(s);
res = expression(s);
next(s);
return res;
}
if (Character.isLowerCase(s.charAt(i))) {
return identifier(s);
}
res.update(new ArrayList<>(), number(s));
return res;
}

private Result identifier(String s) {
Result res = new Result();
StringBuilder sb = new StringBuilder();
while (i < s.length() && Character.isLowerCase(s.charAt(i))) {
sb.append(s.charAt(i));
i++;
}
i--;
next(s);
String variable = sb.toString();
if (evalMap.containsKey(variable)) {
res.update(new ArrayList<>(), evalMap.get(variable));
return res;
} else {
List<String> key = new ArrayList<>();
res.update(key, 1);
return res;
}
}

private int number(String s) {
int res = 0;
while (i < s.length() && s.charAt(i) >= '0' && s.charAt(i) <= '9') {
res = res * 10 + (s.charAt(i) - '0');
i++;
}
i--;
next(s);
return res;
}

private void next(String s) {
i++;
while (i < s.length() && s.charAt(i) == ' ') {
i++;
}
}
``````
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