[Leetcode] Problem 1202 - Smallest String With Swaps

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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

No.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”

No.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”

No.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.

Code

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public class UnionFind {
    private int[] id;
    private int[] size;

    public UnionFind(int n) {
        this.id = new int[n];
        this.size = new int[n];

        for (int i = 0; i < n; i++) {
            id[i] = i;
            size[i] = 1;
        }
    }

    public boolean union(int p, int q) {
        int pRoot = find(p);
        int qRoot = find(q);

        if (pRoot == qRoot)
            return false;

        if (size[pRoot] < size[qRoot]) {
            id[pRoot] = qRoot;
            size[qRoot] += size[pRoot];
        } else {
            id[qRoot] = pRoot;
            size[pRoot] += size[qRoot];
        }

        return true;
    }

    public int find(int p) {
        while (p != id[p])
            p = id[p];

        return p;
    }
}

public String smallestStringWithSwaps(String s, List<List<Integer>> pairs) {
    char[] result = s.toCharArray();
    UnionFind uf = new UnionFind(s.length());
    Map<Integer, List<Integer>> map = new HashMap<>();

    for (List<Integer> pair : pairs)
        uf.union(pair.get(0), pair.get(1));

    for (int i = 0; i < s.length(); i++) {
        int root = uf.find(i);
        map.putIfAbsent(root, new ArrayList<>());
        map.get(root).add(i);
    }

    for (Map.Entry<Integer, List<Integer>> entry : map.entrySet()) {
        List<Integer> index = entry.getValue();
        List<Character> chars = new ArrayList<>();

        for (int idx : index)
            chars.add(result[idx]);

        Collections.sort(chars);

        for (int i = 0; i < index.size(); i++) 
            result[index.get(i)] = chars.get(i);
    }

    return String.valueOf(result);
}