C++ space complexity optimization strategy

C++ Space Complexity Optimization Strategy: Use pointers and references: avoid creating copies and save space. Avoid unnecessary copies: only create copies when needed. Use containers: dynamically allocate and release memory to save space. Use function objects: replace lambda expressions and reduce space usage. Practical example: Optimize the space complexity of a program that counts the number of occurrences of characters in a string by using references.

C++ Space Complexity Optimization Strategy

Space complexity refers to the amount of memory occupied by the program during operation. Optimizing space complexity is crucial to improving program running efficiency. The following are some C++ space complexity optimization strategies:

1. Use pointers

Using pointers can avoid creating copies, thus saving space. For example:

int sum(int* arr, int size) {
  int result = 0;
  for (int i = 0; i < size; i++) {
    result += arr[i];
  }
  return result;
}

2. Use references

References are similar to pointers, but safer. It points directly to the variable, avoiding copying. For example:

int sum(int& a, int& b) {
  return a + b;
}

3. Avoid using unnecessary copies

Unnecessary copies waste space. For example:

// 避免不必要的副本
int x = 10;
int y = x; // 避免创建副本

// 创建副本
int z = x += 2; // 创建副本

4. Using containers

Containers can dynamically allocate and release memory. For example, using a vector container is more space efficient than using an array.

// 使用 vector 容器
vector<int> v;
v.push_back(10);

// 使用数组
int arr[10];
arr[0] = 10;

5. Use function objects

Function objects can replace lambda expressions, thus saving space. For example:

// 使用函数对象
struct Add {
  int operator()(int a, int b) { return a + b; }
};

int sum(int* arr, int size) {
  Add add;
  return accumulate(arr, arr + size, 0, add);
}

Practical case:

Consider a program that counts the number of occurrences of each character in a string. The original implementation is as follows:

unordered_map<char, int> count_characters(string s) {
  unordered_map<char, int> freq;
  for (char c : s) {
    freq[c]++; // 创建副本
  }
  return freq;
}

Using pointers and references can optimize space complexity:

unordered_map<char, int>& count_characters(string s) {
  unordered_map<char, int>& freq = unordered_map<char, int>();
  for (char c : s) {
    freq[&c]++; // 使用引用
  }
  return freq;
}

By using references, we avoid creating copies of string characters, thus saving space.

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