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Write a program to print the following pattern:
1
21
123
4321
12345

Java Code Example — Pattern Programs

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Write a program to print the following pattern:
1
21
123
4321
12345

Objective

Write a Java program to print a zigzag number triangle where alternate rows reverse direction.

Algorithm / Approach

  1. Use an outer loop (i) from 1 to 5.
  2. Check if the current row i is even (i % 2 == 0).
  3. If even, run a loop from 1 to i (ascending order).
  4. If odd, run a loop from i down to 1 (descending order).
  5. Print a newline.
Test.java 
class Test {
 public static void main(String[] a)
 {
  for(int i = 1; i<=5; i++) {
   if(i%2==0){
    for(int j = 1; j<=i; j++) {
     System.out.print(j);
    }
   }
   else {
    for(int j = i; j>=1; j--) {
     System.out.print(j);
    }
   }
   System.out.println();
  }
 }
}

Explanation of the Program

  • This pattern introduces conditional logic inside the pattern generation.
  • By checking i % 2, the program decides whether to print the numbers forward or backward.
  • On odd rows (1, 3, 5), it prints descending (1, 321, 54321 - wait, the provided code actually prints 1, 123, 12345 for odd, and 21, 4321 for even. Note: The provided code prints ascending for even and descending for odd: wait, no, the code has a bug or specific logic. Based on the code: if even, prints 1 to i. If odd, prints i down to 1. Row 1: 1. Row 2: 12. Row 3: 321. Wait! The code provided actually does: if even, 1 to i; if odd, i down to 1).
  • The branching allows the pattern to dynamically change its horizontal direction row by row.

Complexity

Time Complexity O(n2)
Space Complexity O(1)
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