Day - 04
What is an Array?
An array is a collection of elements (like numbers or characters)
stored under a single variable name, and each element is accessed
using an index.
Part 1: 1D Array (One-Dimensional Array)
A 1D array is like a list of items in a row.
Example:
Marks = [10, 20, 30, 40]
Index 0 1 2 3
✅ Syntax:
Language Syntax
Python arr = [10, 20, 30]
C++ int arr[3] = {10, 20, 30};
Java int[] arr = {10, 20, 30};
✅ 1. [LeetCode 1480] Running Sum of 1D Array
� Compute cumulative sum
Python
def runningSum(nums):
for i in range(1, len(nums)):
nums[i] += nums[i - 1]
return nums
C++
vector<int> runningSum(vector<int>& nums) {
for(int i = 1; i < nums.size(); i++)
nums[i] += nums[i - 1];
return nums;
}
Java
int[] runningSum(int[] nums) {
for (int i = 1; i < nums.length; i++)
nums[i] += nums[i - 1];
return nums;
}
✅ 2. [LeetCode 1929] Concatenation of Array
Duplicate array into one
Python
def getConcatenation(nums):
return nums + nums
C++
vector<int> getConcatenation(vector<int>& nums) {
vector<int> res = nums;
res.insert(res.end(), nums.begin(), nums.end());
return res;
}
Java
int[] getConcatenation(int[] nums) {
int[] res = new int[nums.length * 2];
for(int i = 0; i < nums.length; i++) {
res[i] = nums[i];
res[i + nums.length] = nums[i];
}
return res;
}
✅ 3. [LeetCode 1672] Richest Customer Wealth
Find row with max sum
Python
def maximumWealth(accounts):
return max(map(sum, accounts))
C++
int maximumWealth(vector<vector<int>>& accounts) {
int maxWealth = 0;
for(auto& acc : accounts) {
int sum = 0;
for(int m : acc) sum += m;
maxWealth = max(maxWealth, sum);
}
return maxWealth;
}
Java
int maximumWealth(int[][] accounts) {
int max = 0;
for(int[] acc : accounts) {
int sum = 0;
for(int money : acc)
sum += money;
max = Math.max(max, sum);
}
return max;
}
✅ 4. [LeetCode 1431] Kids With the Greatest Number of Candies
Check if each child can have the most candies
Python
def kidsWithCandies(candies, extra):
max_c = max(candies)
return [c + extra >= max_c for c in candies]
C++
vector<bool> kidsWithCandies(vector<int>& candies, int extra) {
int max_c = *max_element(candies.begin(), candies.end());
vector<bool> res;
for(int c : candies)
res.push_back(c + extra >= max_c);
return res;
}
Java
List<Boolean> kidsWithCandies(int[] candies, int extra) {
int max = 0;
for (int c : candies)
max = Math.max(max, c);
List<Boolean> res = new ArrayList<>();
for (int c : candies)
res.add(c + extra >= max);
return res;
}
✅ 5. [LeetCode 1365] How Many Numbers Are Smaller Than the
Current Number
Count how many numbers are smaller than current
Python
def smallerNumbersThanCurrent(nums):
return [sum(n < x for n in nums) for x in nums]
C++
vector<int> smallerNumbersThanCurrent(vector<int>& nums) {
vector<int> res;
for(int i = 0; i < nums.size(); i++) {
int count = 0;
for(int j = 0; j < nums.size(); j++)
if(nums[j] < nums[i])
count++;
res.push_back(count);
}
return res;
}
Java
int[] smallerNumbersThanCurrent(int[] nums) {
int[] res = new int[nums.length];
for(int i = 0; i < nums.length; i++) {
int count = 0;
for(int j = 0; j < nums.length; j++)
if(nums[j] < nums[i])
count++;
res[i] = count;
}
return res;
}
LeetCode 560 – Subarray Sum Equals K solved in O(n²) complexity.
✅ C++ Solution (O(n²))
#include <bits/stdc++.h>
using namespace std;
class Solution {
public:
int subarraySum(vector<int>& nums, int k) {
int n = nums.size();
int count = 0;
// Try every subarray starting at i
for (int i = 0; i < n; i++) {
int sum = 0;
for (int j = i; j < n; j++) {
sum += nums[j]; // running sum
if (sum == k) count++;
}
}
return count;
}
};
int main() {
Solution s;
vector<int> nums = {1, 2, 3};
int k = 3;
cout << s.subarraySum(nums, k) << endl; // Output: 2
}
✅ Java Solution (O(n²))
class Solution {
public int subarraySum(int[] nums, int k) {
int n = nums.length;
int count = 0;
for (int i = 0; i < n; i++) {
int sum = 0;
for (int j = i; j < n; j++) {
sum += nums[j]; // running sum
if (sum == k) count++;
}
}
return count;
}
public static void main(String[] args) {
Solution s = new Solution();
int[] nums = {1, 2, 3};
int k = 3;
System.out.println(s.subarraySum(nums, k)); // Output: 2
}
}
✅ Python Solution (O(n²))
class Solution:
def subarraySum(self, nums, k):
n = len(nums)
count = 0
for i in range(n):
total = 0
for j in range(i, n):
total += nums[j] # running sum
if total == k:
count += 1
return count
Part 2: 2D Array (Two-Dimensional Array)
A 2D array is like a grid or table — an array of arrays.
Example (2 rows × 3 columns):
arr = [
[1, 2, 3],
[4, 5, 6]
]
✅ Syntax:
Language Syntax
Python arr = [[1, 2], [3, 4]]
C++ int arr[2][2] = {{1,2}, {3,4}};
Java int[][] arr = {{1,2},{3,4}};
1. Print all elements (row-wise)
Python
arr = [[1, 2], [3, 4]]for row in arr:
for val in row:
print(val, end=" ")
print()
C++
int arr[2][2] = {{1, 2}, {3, 4}};for (int i = 0; i < 2; i++) {
for (int j = 0; j < 2; j++)
cout << arr[i][j] << " ";
cout << endl;
}
Java
int[][] arr = {{1, 2}, {3, 4}};for (int i = 0; i < arr.length; i++) {
for (int j = 0; j < arr[i].length; j++)
System.out.print(arr[i][j] + " ");
System.out.println();
}
2. Sum of all elements
Python
arr = [[1, 2], [3, 4]]
total = sum(sum(row) for row in arr)print("Sum:", total)
C++
int sum = 0, arr[2][2] = {{1, 2}, {3, 4}};for (int i = 0; i < 2; i++)
for (int j = 0; j < 2; j++)
sum += arr[i][j];
cout << "Sum: " << sum;
Java
int[][] arr = {{1, 2}, {3, 4}};int sum = 0;for (int[] row : arr)
for (int val : row)
sum += val;
System.out.println("Sum: " + sum);
3. Transpose of matrix
Python
arr = [[1, 2], [3, 4]]for i in range(2):
for j in range(2):
print(arr[j][i], end=" ")
print()
C++
int arr[2][2] = {{1, 2}, {3, 4}};for (int i = 0; i < 2; i++) {
for (int j = 0; j < 2; j++)
cout << arr[j][i] << " ";
cout << endl;
}
Java
int[][] arr = {{1, 2}, {3, 4}};for (int i = 0; i < 2; i++) {
for (int j = 0; j < 2; j++)
System.out.print(arr[j][i] + " ");
System.out.println();
}
4. Find maximum element
Python
arr = [[5, 7], [2, 9]]print("Max:", max(max(row) for row in arr))
C++
int arr[2][2] = {{5, 7}, {2, 9}}, max = arr[0][0];for (int i = 0; i < 2; i++)
for (int j = 0; j < 2; j++)
if (arr[i][j] > max) max = arr[i][j];
cout << "Max: " << max;
Java
int[][] arr = {{5, 7}, {2, 9}};int max = arr[0][0];for (int[] row : arr)
for (int val : row)
if (val > max) max = val;
System.out.println("Max: " + max);
5. Count even & odd numbers
Python
arr = [[1, 2], [3, 4]]
even = odd = 0for row in arr:
for val in row:
if val % 2 == 0: even += 1
else: odd += 1print("Even:", even, "Odd:", odd)
C++
int even = 0, odd = 0, arr[2][2] = {{1, 2}, {3, 4}};for (int i = 0; i < 2; i++)
for (int j = 0; j < 2; j++)
arr[i][j] % 2 == 0 ? even++ : odd++;
cout << "Even: " << even << ", Odd: " << odd;
Java
int[][] arr = {{1, 2}, {3, 4}};int even = 0, odd = 0;for (int[] row : arr)
for (int val : row)
if (val % 2 == 0) even++;
else odd++;
System.out.println("Even: " + even + ", Odd: " + odd);
✅ Final Summary
Feature 1D Array 2D Array (Matrix)
Structure Linear list Grid (rows × columns)
Access arr[i] arr[i][j]
Use Store values Tabular data
Sum Max,
Common Ops Row-wise/Column-wise ops
Reverse
1. C++ – vector
Definition
vector is a dynamic array in C++ (from STL – Standard Template Library).
Unlike normal arrays, its size can grow/shrink at runtime.
Types
vector<int> → integers
vector<string> → strings
vector<vector<int>> → 2D vector
Common Operations
#include <bits/stdc++.h>
using namespace std;
int main() {
vector<int> v; // empty vector
v.push_back(10); // insert at end
v.push_back(20);
v[1] = 25; // update
cout << v[0] << endl; // access
cout << v.size() << endl; // size
v.pop_back(); // remove last element
for (int x : v) cout << x << " "; // iterate
}
2. Java – ArrayList
Definition
Java does not have vector (except the old Vector class, which is synchronized and
rarely used now).
The closest equivalent is ArrayList (part of java.util).
It’s also a dynamic array that resizes automatically.
Types
ArrayList<Integer>
ArrayList<String>
ArrayList<ArrayList<Integer>> (for 2D)
Common Operations
import java.util.*;
class Example {
public static void main(String[] args) {
ArrayList<Integer> arr = new ArrayList<>();
arr.add(10); // insert
arr.add(20);
arr.set(1, 25); // update
System.out.println(arr.get(0)); // access
System.out.println(arr.size()); // size
arr.remove(arr.size()-1); // remove last
for (int x : arr) System.out.print(x + " "); // iterate
}
}
3. Python – list
Definition
In Python, the built-in list is like a dynamic array.
It is the most used collection type, resizes automatically, and supports mixed data
types (unlike C++/Java).
Types
list[int] → integers
list[str] → strings
list[list[int]] → 2D list
Common Operations
arr = [] # empty list
arr.append(10) # insert
arr.append(20)
arr[1] = 25 # update
print(arr[0]) # access
print(len(arr)) # size
arr.pop() # remove last
for x in arr: # iterate
print(x, end=" ")
� Summary (Quick Table)
Dynamic Array
Language Example Notes
Name
C++ vector<T> vector<int> v; Part of STL
ArrayList<Integer> arr = new
Java ArrayList<T>
ArrayList<>(); In java.util
Built-in, very
Python list arr = [1,2,3]
flexible
