1202. Smallest String With Swaps

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LeetCode

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Problem

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.

Code

class Solution {
    public class UnionFind {
        int[] parents;
        int[] ranks;

        public UnionFind(int n) {
            parents = new int[n];
            ranks = new int[n];

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

        public boolean union(int x, int y) {
            int xParent = find(x);
            int yParent = find(y);

            if (xParent == yParent) return false;

            if (ranks[xParent] > ranks[yParent]) {
                parents[yParent] = xParent;
            } else if (ranks[xParent] < ranks[yParent]) {
                parents[xParent] = yParent;
            } else {
                parents[yParent] = xParent;
                ranks[xParent]++;
            }

            return true;
        }

        public int find(int node) {
            if(node == parents[node]) return node;
            return find(parents[node]);
        }
    }

    public String smallestStringWithSwaps(String s, List<List<Integer>> pairs) {
        UnionFind node = new UnionFind(s.length());
        for(List<Integer> pair : pairs) {
            node.union(pair.get(0), pair.get(1));
        }

        Map<Integer, PriorityQueue<Character>> map = new HashMap<>();

        for (int i = 0; i < s.length(); i++) {
            int parent = node.find(i);
            if(!map.containsKey(parent)) {
                map.put(parent, new PriorityQueue<>());
            }
            map.get(parent).offer(s.charAt(i));
        }

        StringBuilder res = new StringBuilder();
        for (int i = 0; i < s.length(); i++) {
            res.append(map.get(node.find(i)).poll());
        }

        return res.toString();
    }
}