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Problem solution · C++

Sort Array Using Prefix Reversals

Sort Array Using Prefix Reversals: a C++ solution using breadth-first search. Learn the idea, check the complexity, and read the full code, with credit to Kamyu LeetCode Solutions.

Technique
Breadth-first search
Source
Kamyu LeetCode Solutions
Length
93 lines
Start with the idea.

Try the problem first. If you get stuck, read the approach below, then write your own solution. The full code is at the bottom.

Approach

Breadth-first search

For Sort Array Using Prefix Reversals, the implementation explores reachable states in layers, which is the standard shape for unweighted shortest paths and minimum-step transitions.

  1. Model each valid configuration as a state and each legal move as an edge.
  2. Seed the queue with the starting state and mark it immediately.
  3. Expand each state once, recording distance or reachability for unseen neighbours.

Code notes

  • 93 lines of C++ from the credited upstream file sort-array-using-prefix-reversals.cpp.
  • The implementation visibly relies on sequence storage, hash lookup.
  • 9 loop blocks detected.

Complexity

Verify that each state and transition is processed only a bounded number of times; that determines the traversal cost.

Check the problem constraints before deciding whether this complexity will pass.

Source

Code and credit

This code comes from Kamyu LeetCode Solutions by kamyu104 and is used under the MIT licence.

Open problem ↗Original source ↗
Full codeSort Array Using Prefix Reversals · C++C++
Use this to learn the idea, then write your own version.
// Time:  O(n! * n * m)// Space: O(n! * n) // bi-bfsclass Solution {public:    int sortArray(vector<int>& nums, vector<int>& pre) {        const auto& to_string = [](const auto& arr) {            string result;            result.reserve(size(arr));            for (const auto& x : arr) {                result.push_back('0' + x);            }            return result;        };         const auto& bi_bfs = [&](const auto& start, const auto& target) {            unordered_set<string> left = {start}, right = {target}, lookup;            for (int steps = 0; !empty(left); ++steps) {                if (size(left) > size(right)) {                    swap(left, right);                }                for (const auto& x : left) {                    lookup.emplace(x);                }                unordered_set<string> new_left;                for (auto x : left) {                    if (right.count(x)) {                        return steps;                    }                    for (const auto& i : pre) {                        reverse(begin(x), begin(x) + i);                        if (!lookup.count(x)) {                            new_left.emplace(x);                        }                        reverse(begin(x), begin(x) + i);                    }                }                left = move(new_left);            }            return -1;        };         vector<int> arr(size(nums));        iota(begin(arr), end(arr), 0);        return bi_bfs(to_string(nums), to_string(arr));    }}; // Time:  O(n! * n * m)// Space: O(n! * n)// bfsclass Solution2 {public:    int sortArray(vector<int>& nums, vector<int>& pre) {        const auto& to_string = [](const auto& arr) {            string result;            result.reserve(size(arr));            for (const auto& x : arr) {                result.push_back('0' + x);            }            return result;        };         const auto& bfs = [&](const auto& start, const auto& target) {            unordered_set<string> lookup = {start};            vector<string> q = {start};            for (int steps = 0; !empty(q); ++steps) {                vector<string> new_q;                for (auto x : q) {                    if (x == target) {                        return steps;                    }                    for (const auto& i : pre) {                        reverse(begin(x), begin(x) + i);                        if (!lookup.count(x)) {                            lookup.emplace(x);                            new_q.emplace_back(x);                        }                        reverse(begin(x), begin(x) + i);                    }                }                q = move(new_q);            }            return -1;        };         vector<int> arr(size(nums));        iota(begin(arr), end(arr), 0);        return bfs(to_string(nums), to_string(arr));    }};

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