MediumBacktracking

Letter Combinations Phone Number

Letter combinations of phone number

Solution Approach

Recursively explore all possible solutions. Make choice, explore recursively, undo choice (backtrack). Use when searching permutations, combinations, or paths.

Implementation

def permute(nums):
    result = []
    def backtrack(path, remaining):
        if not remaining:
            result.append(path)
            return
        for i in range(len(remaining)):
            backtrack(path + [remaining[i]], remaining[:i] + remaining[i+1:])
    backtrack([], nums)
    return result

Complexity Analysis

Time Complexity

O(n!)

Space Complexity

O(n)

Key Learning Points

Recursive explorationUndo/backtrack stepUseful for all permutations/combinations