Binary TreesBinary Trees36
❯
  1. 1Preorder Traversal of a Binary Tree using Recursion
  2. 2Preorder Traversal of a Binary Tree using Iteration
  3. 3Inorder Traversal of a Binary Tree using Recursion
  4. 4Inorder Traversal of a Binary Tree using Iteration
  5. 5Postorder Traversal of a Binary Tree Using Recursion
  6. 6Postorder Traversal of a Binary Tree using Iteration
  7. 7Level Order Traversal of a Binary Tree using Recursion
  8. 8Level Order Traversal of a Binary Tree using Iteration
  9. 9Reverse Level Order Traversal of a Binary Tree using Iteration
  10. 10Reverse Level Order Traversal of a Binary Tree using Recursion
  11. 11Find Height of a Binary Tree
  12. 12Find Diameter of a Binary Tree
  13. 13Find Mirror of a Binary Tree
  14. 14Left View of a Binary Tree
  15. 15Right View of a Binary Tree
  16. 16Top View of a Binary Tree
  17. 17Bottom View of a Binary Tree
  18. 18Zigzag Traversal of a Binary Tree
  19. 19Check if a Binary Tree is Balanced
  20. 20Diagonal Traversal of a Binary Tree
  21. 21Boundary Traversal of a Binary Tree
  22. 22Construct a Binary Tree from a String with Bracket Representation
  23. 23Convert a Binary Tree into a Doubly Linked List
  24. 24Convert a Binary Tree into a Sum Tree
  25. 25Find Minimum Swaps Required to Convert a Binary Tree into a BST
  26. 26Check if a Binary Tree is a Sum Tree
  27. 27Check if All Leaf Nodes are at the Same Level in a Binary Tree
  28. 28Lowest Common Ancestor (LCA) in a Binary Tree
  29. 29Solve the Tree Isomorphism Problem
  30. 30Check if a Binary Tree Contains Duplicate Subtrees of Size 2 or More
  31. 31Check if Two Binary Trees are Mirror Images
  32. 32Calculate the Sum of Nodes on the Longest Path from Root to Leaf in a Binary Tree
  33. 33Print All Paths in a Binary Tree with a Given Sum
  34. 34Find the Distance Between Two Nodes in a Binary Tree
  35. 35Find the kth Ancestor of a Node in a Binary Tree
  36. 36Find All Duplicate Subtrees in a Binary Tree
GraphsGraphs46
❯
  1. 1Breadth-First Search in Graphs
  2. 2Depth-First Search in Graphs
  3. 3Number of Provinces in an Undirected Graph
  4. 4Connected Components in a Matrix
  5. 5Rotten Oranges Problem - BFS in Matrix
  6. 6Flood Fill Algorithm - Graph Based
  7. 7Detect Cycle in an Undirected Graph using DFS
  8. 8Detect Cycle in an Undirected Graph using BFS
  9. 9Distance of Nearest Cell Having 1 - Grid BFS
  10. 10Surrounded Regions in Matrix using Graph Traversal
  11. 11Number of Enclaves in Grid
  12. 12Word Ladder - Shortest Transformation using Graph
  13. 13Word Ladder II - All Shortest Transformation Sequences
  14. 14Number of Distinct Islands using DFS
  15. 15Check if a Graph is Bipartite using DFS
  16. 16Topological Sort Using DFS
  17. 17Topological Sort using Kahn's Algorithm
  18. 18Cycle Detection in Directed Graph using BFS
  19. 19Course Schedule - Task Ordering with Prerequisites
  20. 20Course Schedule 2 - Task Ordering Using Topological Sort
  21. 21Find Eventual Safe States in a Directed Graph
  22. 22Alien Dictionary Character Order
  23. 23Shortest Path in Undirected Graph with Unit Distance
  24. 24Shortest Path in DAG using Topological Sort
  25. 25Dijkstra's Algorithm Using Set - Shortest Path in Graph
  26. 26Dijkstra’s Algorithm Using Priority Queue
  27. 27Shortest Distance in a Binary Maze using BFS
  28. 28Path With Minimum Effort in Grid using Graphs
  29. 29Cheapest Flights Within K Stops - Graph Problem
  30. 30Number of Ways to Reach Destination in Shortest Time - Graph Problem
  31. 31Minimum Multiplications to Reach End - Graph BFS
  32. 32Bellman-Ford Algorithm for Shortest Paths
  33. 33Floyd Warshall Algorithm for All-Pairs Shortest Path
  34. 34Find the City With the Fewest Reachable Neighbours
  35. 35Minimum Spanning Tree in Graphs
  36. 36Prim's Algorithm for Minimum Spanning Tree
  37. 37Disjoint Set (Union-Find) with Union by Rank and Path Compression
  38. 38Kruskal's Algorithm - Minimum Spanning Tree
  39. 39Minimum Operations to Make Network Connected
  40. 40Most Stones Removed with Same Row or Column
  41. 41Accounts Merge Problem using Disjoint Set Union
  42. 42Number of Islands II - Online Queries using DSU
  43. 43Making a Large Island Using DSU
  44. 44Bridges in Graph using Tarjan's Algorithm
  45. 45Articulation Points in Graphs
  46. 46Strongly Connected Components using Kosaraju's Algorithm

Implement Atoi (String to Integer Conversion)

Strings

Contents

ProblemExamplesSolutionAlgorithmCodeTimeSpace

Problem Statement❯

You are given a string that may contain a number. Your task is to convert this string into a valid integer, just like the C/C++ function atoi() does.

However, the conversion should follow these rules:

  • Ignore leading whitespaces.
  • The first non-whitespace character may be '+' or '-' to indicate sign.
  • Only read the digits after the optional sign.
  • If any non-digit character appears after the digits, stop reading further.
  • If the converted number exceeds the 32-bit signed integer range ([-231, 231-1]), clamp it to the nearest bound.

If the string does not contain a valid number, return 0.

Examples❯

Input String Output Description
"42" 42 Normal case with valid positive number
" -42" -42 Leading spaces and negative sign handled
"4193 with words" 4193 Number ends before non-digit characters
"words and 987" 0 Invalid as it doesn't start with a number
"+123" 123 Valid positive number with explicit sign
"-91283472332" -2147483648 Below 32-bit signed int range, clamped to INT_MIN
"91283472332" 2147483647 Above 32-bit signed int range, clamped to INT_MAX
"" 0 Empty string returns 0
" " 0 Whitespace only returns 0
"-" 0 Only sign with no digits returns 0
"+0 123" 0 Only 0 is read before space, rest ignored

Solution❯

To convert a string into an integer (like the atoi function), we need to carefully interpret the string one character at a time. It's not just about parsing digits — we must also think about signs, spaces, invalid characters, and potential overflows.

Let’s break it down for a beginner:

1. Ignore leading whitespaces: The first thing we do is skip any space characters at the beginning of the string. These should not affect the result.

2. Check for optional '+' or '-' sign: After trimming spaces, if the first character is '+' or '-', we store this to determine the final sign of our number.

3. Convert following digits: Next, we move character by character and as long as we see digits (0-9), we build the number by multiplying the existing result by 10 and adding the new digit.

4. Stop on invalid character: If we hit a character that's not a digit (e.g., letter, punctuation, space), we stop immediately. Only the valid prefix of the string is considered.

5️⃣ Handle overflow: While building the number, if at any point it goes beyond the limits of a 32-bit signed integer, we stop and clamp the result to:

  • 2147483647 if the number is too large (positive overflow)
  • -2147483648 if the number is too small (negative overflow)

6️⃣ Empty or invalid string: If after removing spaces and optional sign we find nothing to convert, or the string starts with something invalid, we simply return 0.

Example thoughts:

  • "42" → clean numeric string, converts to 42.
  • " -42" → spaces skipped, '-' sign handled, digits converted → -42.
  • "words and 987" → starts with non-digit → invalid → return 0.
  • "91283472332" → valid number but too large → clamp to 2147483647.
  • "" or " " → empty after trimming → return 0.

This approach ensures correctness and protects against errors, making it suitable for real-world use cases like parsing user input.

Algorithm Steps❯

  1. Trim leading whitespaces from the input string
  2. Check if the next character is '+' or '-'. Store the sign
  3. Iterate through each character:
  4. → If it's a digit, convert and add to result
  5. → If not a digit, break the loop
  6. → Before each step, check if result will overflow/underflow
  7. Return result multiplied by sign

Code

Java
Python
JavaScript
C
C++
C#
Kotlin
Swift
Go
Php
public class AtoiConverter {
    public static int myAtoi(String s) {
        if (s == null || s.isEmpty()) return 0;
        s = s.trim(); // remove leading/trailing spaces
        if (s.isEmpty()) return 0;

        int sign = 1, i = 0, result = 0;
        int INT_MAX = Integer.MAX_VALUE;
        int INT_MIN = Integer.MIN_VALUE;

        if (s.charAt(i) == '+' || s.charAt(i) == '-') {
            sign = (s.charAt(i) == '-') ? -1 : 1;
            i++;
        }

        while (i < s.length() && Character.isDigit(s.charAt(i))) {
            int digit = s.charAt(i) - '0';

            if (result > (INT_MAX - digit) / 10) {
                return (sign == 1) ? INT_MAX : INT_MIN;
            }

            result = result * 10 + digit;
            i++;
        }

        return result * sign;
    }

    public static void main(String[] args) {
        String input = "   -42";
        System.out.println("Converted Integer: " + myAtoi(input));
    }
}

Time Complexity❯

CaseTime ComplexityExplanation
Best CaseO(1)When the string is empty or contains only whitespaces, the function exits early.
Average CaseO(n)Where n is the number of characters to parse through until a non-digit or end is encountered.
Worst CaseO(n)Involves scanning through the entire valid digit portion of the string for parsing.

Space Complexity❯

O(1)

Explanation: Only a fixed number of variables are used for computation, regardless of input size.