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Bars of width 1 have the given heights. After rain, water collects above each bar up to the lower of the tallest bars on its left and right. Compute the total trapped water. An O(n) two-pointer solution is expected.
Implement trapped_water(heights).
Input
heights: Bar heights from left to right.
Output
Return the total units of water.
Constraints
0 ≤ length ≤ 200,000; 0 ≤ height ≤ 100,000
Examples
Example 1
Input:
heights = [0, 1, 0, 2, 1, 0, 1, 3, 2, 1, 2, 1]
Expected output:
6Explanation
Water fills the dips: 1 + 1 + 2 + 1 + 1 = 6 units.
Example 2
Input:
heights = [3, 2, 1]
Expected output:
0Explanation
Heights only go down, so water runs off: 0.
References
-
Example 1 Not runInput:
heights = [0, 1, 0, 2, 1, 0, 1, 3, 2, 1, 2, 1]Expected output:6Your output -
Example 2 Not runInput:
heights = [3, 2, 1]Expected output:0Your output
5 hidden tests also run when you submit.
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💻 About This C++ Online Executor
The CodeUtility C++ Executor lets you write and run C++ code directly in your browser - no installation or compiler setup needed. It’s powered by a secure sandbox that supports real C++ standards from C++11, C++14, C++17, up to the latest version.
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💡 C++ Basics & Examples You Can Try Above
1. Declaring Variables and Constants
C++ requires you to declare the type of each variable. Use const to define read-only values that cannot be changed after initialization.
int age = 30;
double pi = 3.14159;
char grade = 'A';
std::string name = "Alice";
bool isActive = true;
// Constants
const int MAX_USERS = 100;
const std::string COMPANY = "CodeUtility";
// Attempting to modify a const will cause a compile-time error
// MAX_USERS = 200; // ❌ error
2. Conditionals (if / switch)
Use if, else if, else for branching. Use switch for multiple values.
int x = 2;
if (x == 1) {
std::cout << "One\n";
} else if (x == 2) {
std::cout << "Two\n";
} else {
std::cout << "Other\n";
}
// Switch-case
switch (x) {
case 1:
std::cout << "One\n";
break;
case 2:
std::cout << "Two\n";
break;
default:
std::cout << "Other\n";
}3. Loops
Use for, while, and do-while to repeat code blocks.
for (int i = 0; i < 3; i++) {
std::cout << i << "\n";
}
int n = 3;
while (n > 0) {
std::cout << n << "\n";
n--;
}4. Arrays
Arrays store multiple elements of the same type.
int numbers[3] = {10, 20, 30};
std::cout << numbers[1] << "\n";5. Array/Vector Manipulation
Use std::vector for dynamic arrays.
#include <vector>
std::vector nums = {1, 2, 3};
nums.push_back(4);
nums.pop_back();
for (int n : nums) {
std::cout << n << " ";
} 6. Console Input/Output
Use std::cin for input and std::cout for output.
std::string name;
std::cout << "Enter your name: ";
std::cin >> name;
std::cout << "Hello, " << name << "\n";7. Functions
Functions group reusable logic. Use parameters and return types.
int add(int a, int b) {
return a + b;
}
std::cout << add(3, 4);8. Maps
std::map stores key-value pairs like dictionaries.
#include <map>
std::map ages;
ages["Alice"] = 30;
std::cout << ages["Alice"]; 9. Exception Handling
Use try and catch to handle runtime errors.
try {
throw std::runtime_error("Error occurred");
} catch (const std::exception& e) {
std::cout << e.what();
}10. File I/O
Use fstream to read from and write to files.
#include <fstream>
std::ofstream out("file.txt");
out << "Hello File";
out.close();
std::ifstream in("file.txt");
std::string line;
getline(in, line);
std::cout << line;
in.close();11. String Manipulation
C++ strings support methods like length(), substr(), and find().
std::string text = "Hello World";
std::cout << text.length();
std::cout << text.substr(0, 5);
std::cout << text.find("World");12. Classes & Objects
C++ supports object-oriented programming using classes.
class Person {
public:
std::string name;
Person(std::string n) : name(n) {}
void greet() { std::cout << "Hi, I'm " << name; }
};
Person p("Alice");
p.greet();13. Pointers
A pointer stores the memory address of another variable. Use & to get an address,
and * to dereference (access the value at the address).
int x = 10;
int* ptr = &x;
std::cout << "Value of x: " << x << "\n";
std::cout << "Address of x: " << ptr << "\n";
std::cout << "Value from pointer: " << *ptr << "\n";
// Modify x through pointer
*ptr = 20;
std::cout << "Updated x: " << x << "\n";🐞 How to Debug %{language} Code Online
Use the online %{language} debugger to find runtime and logic errors without installing a compiler or desktop IDE. Server mode compiles with debug symbols and runs the program under GDB.
- Set one or more breakpoints beside the line numbers.
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- Inspect the current line, call stack, and variables.
- Use Continue, Step Over, Step Into, or Step Out to control execution.
- Select Stop to end the debugging session.
Start with a small reproducible example and place a breakpoint just before the value or control flow becomes unexpected.