LeetCode 867: Transpose Matrix

Meta Data

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Difficulty: easy First Attempt: 2022-09-17

• Submissions: Runtime Error ×2 → Accepted ×3

Approach

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Transposing swaps rows and columns (Matrix Template 1): an m × n matrix becomes n × m with res[j][i] = matrix[i][j]. The outer loop runs over the new rows (old columns), the inner over the new columns (old rows). Python’s zip(*matrix) is exactly the transpose, with tuples as rows.

My Attempts

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The first two swapped the loop ranges: attempt 1 even passed matrix[0] (a list) to range, and attempt 2 used len(matrix[0]) but with inner and outer reversed, so the non-square [[5, 0]] went out of range. A one-line zip(*matrix) passed.

Solution

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class Solution:
    def transpose(self, matrix: List[List[int]]) -> List[List[int]]:
        m, n = len(matrix), len(matrix[0])
        return [[matrix[i][j] for i in range(m)] for j in range(n)]

Complexity

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Time O(mn), space O(mn) (output).

Mistakes to Avoid

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• The matrix need not be square; size both loops to the new shape.

Interview Explanation

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Transposing swaps rows and columns, so I build an n × m matrix whose row j, column i holds the original row i, column j. Time O(mn).