<p>You are given a string <code>s</code>, and an array of pairs of indices in the string&nbsp;<code>pairs</code>&nbsp;where&nbsp;<code>pairs[i] =&nbsp;[a, b]</code>&nbsp;indicates 2 indices(0-indexed) of the string.</p>

<p>You can&nbsp;swap the characters at any pair of indices in the given&nbsp;<code>pairs</code>&nbsp;<strong>any number of times</strong>.</p>

<p>Return the&nbsp;lexicographically smallest string that <code>s</code>&nbsp;can be changed to after using the swaps.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>

<pre>
<strong>Input:</strong> s = &quot;dcab&quot;, pairs = [[0,3],[1,2]]
<strong>Output:</strong> &quot;bacd&quot;
<strong>Explaination:</strong> 
Swap s[0] and s[3], s = &quot;bcad&quot;
Swap s[1] and s[2], s = &quot;bacd&quot;
</pre>

<p><strong class="example">Example 2:</strong></p>

<pre>
<strong>Input:</strong> s = &quot;dcab&quot;, pairs = [[0,3],[1,2],[0,2]]
<strong>Output:</strong> &quot;abcd&quot;
<strong>Explaination: </strong>
Swap s[0] and s[3], s = &quot;bcad&quot;
Swap s[0] and s[2], s = &quot;acbd&quot;
Swap s[1] and s[2], s = &quot;abcd&quot;</pre>

<p><strong class="example">Example 3:</strong></p>

<pre>
<strong>Input:</strong> s = &quot;cba&quot;, pairs = [[0,1],[1,2]]
<strong>Output:</strong> &quot;abc&quot;
<strong>Explaination: </strong>
Swap s[0] and s[1], s = &quot;bca&quot;
Swap s[1] and s[2], s = &quot;bac&quot;
Swap s[0] and s[1], s = &quot;abc&quot;
</pre>

<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li><code>1 &lt;= s.length &lt;= 10^5</code></li>
	<li><code>0 &lt;= pairs.length &lt;= 10^5</code></li>
	<li><code>0 &lt;= pairs[i][0], pairs[i][1] &lt;&nbsp;s.length</code></li>
	<li><code>s</code>&nbsp;only contains lower case English letters.</li>
</ul>


class Solution {
public:
    typedef vector<vector<int>> Graph ;
    void add_undirected_edge (Graph & graph,int from , int to)
    {
        graph[from].push_back(to);
        graph[to].push_back(from);
    }
    void dfs (Graph& graph , int node , vector<bool>&visited,vector<int>& c_index)
    {
        visited[node] = 1 ;
        c_index.push_back(node);
        for (auto neighbour : graph[node])
            if (!visited[neighbour])dfs(graph,neighbour,visited,c_index);
    }
    string smallestStringWithSwaps(string s, vector<vector<int>>& pairs) {
        int nodes = s.size();
        // build graph 
        Graph graph (nodes);
        for(auto pairr : pairs)
            add_undirected_edge (graph,pairr[0],pairr[1]);
        //--------
        vector<bool> visited (nodes,0);
        for (int i = 0 ; i < nodes;i++)
        {
            if (!visited[i]&& graph[i].size()>0)
            {
                vector<int> c_index ;
                dfs(graph,i,visited,c_index);
                vector<char> c_char ;
                for (auto index : c_index)
                    c_char.push_back(s[index]);
                sort(c_index.begin(),c_index.end());
                sort(c_char.begin(),c_char.end());
                int j = 0 ;
                for (auto index : c_index)
                    s[index] = c_char[j] , j++ ;
            }
        }
    return s ;
    }
};
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