74. Search a 2D Matrix - Leetcode Solution

LeetCode problem Solution code

đź’ˇ Step-by-Step Thought Process

Brute Force

  1. Understand the problem: Search for a target value in an m x n matrix where each row is sorted and the first element of each row is greater than the last element of the previous row.
  2. Iterate through each row in the matrix.
  3. For each row, check if the target exists in that row using a linear search or Python's 'in' operator.
  4. If the target is found in any row, return True.
  5. If the target is not found after checking all rows, return False.

Code Solution (Brute Force)

# Brute Force Solution
class Solution:
    def searchMatrix(self, matrix: List[List[int]], target: int) -> bool:
        for row in matrix:
            if target in row:
                return True
        
        return False
# Time: O(m * n)
# Space: O(1)

#include <vector>
using namespace std;

class Solution {
public:
    bool searchMatrix(vector<vector<int>>& matrix, int target) {
        for (const auto& row : matrix) {
            for (int num : row) {
                if (num == target) {
                    return true;
                }
            }
        }
        return false;
    }
};
// Time: O(m * n)
// Space: O(1)
class Solution {
    public boolean searchMatrix(int[][] matrix, int target) {
        for (int[] row : matrix) {
            for (int num : row) {
                if (num == target) {
                    return true;
                }
            }
        }
        return false;
    }
}
// Time: O(m * n)
// Space: O(1)
var searchMatrix = function(matrix, target) {
    for (const row of matrix) {
        for (const num of row) {
            if (num === target) {
                return true;
            }
        }
    }
    return false;
};
// Time: O(m * n)
// Space: O(1)

Why the Brute-Force Solution is Inefficient

The brute-force solution checks every element in each row sequentially, leading to:

Optimal Solution

The optimal solution treats the matrix as a flattened sorted array and uses binary search, achieving O(log(m * n)) time complexity:

  1. Get the dimensions m (rows) and n (columns) of the matrix, and compute the total elements t as m * n.
  2. Initialize two pointers, l to 0 and r to t-1, for binary search.
  3. While l is less than or equal to r, compute the middle index mid as (l + r) // 2.
  4. Convert mid to matrix coordinates: row (mid_i) as mid // n and column (mid_j) as mid % n.
  5. Retrieve the middle element mid_num as matrix[mid_i][mid_j].
  6. If mid_num equals the target, return True.
  7. If mid_num is greater than the target, adjust r to mid - 1 to search the left half.
  8. If mid_num is less than the target, adjust l to mid + 1 to search the right half.
  9. Return False if the target is not found.

Code Solution (Optimal)

# Optimal Solution
class Solution:
    def searchMatrix(self, matrix: List[List[int]], target: int) -> bool:
        m = len(matrix)
        n = len(matrix[0])
        t = m * n
        l = 0
        r = t - 1

        while l <= r:
            mid = (l + r) // 2
            mid_i = mid // n
            mid_j = mid % n
            mid_num = matrix[mid_i][mid_j]

            if target == mid_num:
                return True
            elif target < mid_num:
                r = mid - 1
            else:
                l = mid + 1

        return False

# Time Complexity: O(log(m * n))
# Space Complexity: O(1)

class Solution {
public:
    bool searchMatrix(vector<vector<int>>& matrix, int target) {
        int m = matrix.size();
        int n = matrix[0].size();
        int l = 0;
        int r = m * n - 1;

        while (l <= r) {
            int mid = (l + r) / 2;
            int i = mid / n;
            int j = mid % n;
            int midNum = matrix[i][j];

            if (target == midNum) {
                return true;
            } else if (target < midNum) {
                r = mid - 1;
            } else {
                l = mid + 1;
            }
        }

        return false;
    }
};

public class Solution {
    public boolean searchMatrix(int[][] matrix, int target) {
        int m = matrix.length;
        int n = matrix[0].length;
        int l = 0;
        int r = m * n - 1;

        while (l <= r) {
            int mid = (l + r) / 2;
            int i = mid / n;
            int j = mid % n;
            int midNum = matrix[i][j];

            if (target == midNum) {
                return true;
            } else if (target < midNum) {
                r = mid - 1;
            } else {
                l = mid + 1;
            }
        }

        return false;
    }
}

var searchMatrix = function(matrix, target) {
    let m = matrix.length;
    let n = matrix[0].length;
    let l = 0;
    let r = m * n - 1;

    while (l <= r) {
        let mid = Math.floor((l + r) / 2);
        let i = Math.floor(mid / n);
        let j = mid % n;
        let midNum = matrix[i][j];

        if (target === midNum) {
            return true;
        } else if (target < midNum) {
            r = mid - 1;
        } else {
            l = mid + 1;
        }
    }

    return false;
};

Detailed Explanation

Understanding the Problem: Search a 2D Matrix

The “Search a 2D Matrix” problem asks us to efficiently search for a target value in a 2D matrix that has the following sorted properties:

  • Each row is sorted in ascending order.
  • The first integer of each row is greater than the last integer of the previous row.

This structure allows us to treat the 2D matrix almost like a 1D sorted array.

Example:

  • Input: matrix = [[1,3,5,7],[10,11,16,20],[23,30,34,60]], target = 3
  • Output: true

Why This Problem Matters

This problem is a hybrid of matrix traversal and binary search, testing your ability to convert between 2D and 1D indexing. It's frequently asked in coding interviews and serves as a foundation for more complex matrix and search problems.

Optimal Approach: Binary Search Over Flattened Matrix

Because of the matrix’s sorted structure across rows and columns, we can apply a binary search as if the matrix were a single flat array. The total number of elements is m * n, and we can use math to translate flat indices to 2D coordinates.

Steps:

  1. Let m be the number of rows and n be the number of columns.
  2. Initialize binary search with left = 0 and right = m * n - 1.
  3. While left <= right:
    • Compute the middle index: mid = Math.floor((left + right) / 2).
    • Map mid to 2D coordinates:
      • row = Math.floor(mid / n)
      • col = mid % n
    • Get the element: matrix[row][col].
    • If the element equals the target, return true.
    • If it's less than the target, search the right half: left = mid + 1.
    • If it's greater than the target, search the left half: right = mid - 1.
  4. If the loop ends, return false — the target does not exist in the matrix.

Example Walkthrough

Input: matrix = [[1,3,5,7],[10,11,16,20],[23,30,34,60]], target = 16
m = 3, n = 4 → total = 12 elements
Initial range: left = 0, right = 11

  • mid = 5 → matrix[1][1] = 11 → 11 < 16 → move left to 6
  • mid = 8 → matrix[2][0] = 23 → 23 > 16 → move right to 7
  • mid = 6 → matrix[1][2] = 16 → match → return true

Time and Space Complexity

Time Complexity: O(log(m * n)), where m is rows and n is columns. This is due to binary search over all elements.
Space Complexity: O(1), as the algorithm uses only constant extra memory.

Edge Cases to Consider

  • Empty matrix → return false
  • Single row or column → should still apply binary search properly
  • Target smaller than the smallest element or larger than the largest → return false early

Conclusion

The “Search a 2D Matrix” problem teaches how binary search can be applied beyond 1D arrays by mapping indices across dimensions. This approach is both time-efficient and elegant, making it ideal for large datasets structured in tabular formats.

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