1202. Smallest String With Swaps

1202. Smallest String With Swaps (Medium)

You are given a string s, and an array of pairs of indices in the string pairs where pairs[i] = [a, b] indicates 2 indices(0-indexed) of the string.

You can swap the characters at any pair of indices in the given pairs any number of times.

Return the lexicographically smallest string that s can be changed to after using the swaps.

 

Example 1:

Input: s = "dcab", pairs = [[0,3],[1,2]]
Output: "bacd"
Explaination: 
Swap s[0] and s[3], s = "bcad"
Swap s[1] and s[2], s = "bacd"

Example 2:

Input: s = "dcab", pairs = [[0,3],[1,2],[0,2]]
Output: "abcd"
Explaination: 
Swap s[0] and s[3], s = "bcad"
Swap s[0] and s[2], s = "acbd"
Swap s[1] and s[2], s = "abcd"

Example 3:

Input: s = "cba", pairs = [[0,1],[1,2]]
Output: "abc"
Explaination: 
Swap s[0] and s[1], s = "bca"
Swap s[1] and s[2], s = "bac"
Swap s[0] and s[1], s = "abc"

 

Constraints:

• 1 <= s.length <= 10^5
• 0 <= pairs.length <= 10^5
• 0 <= pairs[i][0], pairs[i][1] < s.length
• s only contains lower case English letters.

Related Topics:
Array, Union Find

Solution 1. Union Find

// OJ: https://leetcode.com/problems/smallest-string-with-swaps/
// Author: github.com/lzl124631x
// Time: O(NlogN)
// Space: O(N)
class Solution {
    vector<int> id;
    int find(int x) {
        return id[x] == x ? x : (id[x] = find(id[x]));
    }
    void connect(int x, int y) {
        int p = find(x), q = find(y);
        id[p] = q;
    }
public:
    string smallestStringWithSwaps(string s, vector<vector<int>>& A) {
        int N = s.size();
        id.assign(N, 0);
        iota(begin(id), end(id), 0);
        for (auto &p : A) connect(p[0], p[1]);
        unordered_map<int, vector<int>> m;
        for (int i = 0; i < N; ++i) m[find(i)].push_back(i);
        for (auto &[k, v] : m) sort(begin(v), end(v), [&](int a, int b) { return s[a] > s[b]; });
        string ans;
        for (int i = 0; i < N; ++i) {
            ans += s[m[find(i)].back()];
            m[find(i)].pop_back();
        }
        return ans;
    }
};