Minimum Adjacent Swaps to Reach the Kth Smallest Number
<p>You are given a string <code>num</code>, representing a large integer, and an integer <code>k</code>.</p>

<p>We call some integer <strong>wonderful</strong> if it is a <strong>permutation</strong> of the digits in <code>num</code> and is <strong>greater in value</strong> than <code>num</code>. There can be many wonderful integers. However, we only care about the <strong>smallest-valued</strong> ones.</p>

<ul>
	<li>For example, when <code>num = &quot;5489355142&quot;</code>:

    <ul>
    	<li>The 1<sup>st</sup> smallest wonderful integer is <code>&quot;5489355214&quot;</code>.</li>
    	<li>The 2<sup>nd</sup> smallest wonderful integer is <code>&quot;5489355241&quot;</code>.</li>
    	<li>The 3<sup>rd</sup> smallest wonderful integer is <code>&quot;5489355412&quot;</code>.</li>
    	<li>The 4<sup>th</sup> smallest wonderful integer is <code>&quot;5489355421&quot;</code>.</li>
    </ul>
    </li>

</ul>

<p>Return <em>the <strong>minimum number of adjacent digit swaps</strong> that needs to be applied to </em><code>num</code><em> to reach the </em><code>k<sup>th</sup></code><em><strong> smallest wonderful</strong> integer</em>.</p>

<p>The tests are generated in such a way that <code>k<sup>th</sup></code>&nbsp;smallest wonderful integer exists.</p>

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

<pre>
<strong>Input:</strong> num = &quot;5489355142&quot;, k = 4
<strong>Output:</strong> 2
<strong>Explanation:</strong> The 4<sup>th</sup> smallest wonderful number is &quot;5489355421&quot;. To get this number:
- Swap index 7 with index 8: &quot;5489355<u>14</u>2&quot; -&gt; &quot;5489355<u>41</u>2&quot;
- Swap index 8 with index 9: &quot;54893554<u>12</u>&quot; -&gt; &quot;54893554<u>21</u>&quot;
</pre>

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

<pre>
<strong>Input:</strong> num = &quot;11112&quot;, k = 4
<strong>Output:</strong> 4
<strong>Explanation:</strong> The 4<sup>th</sup> smallest wonderful number is &quot;21111&quot;. To get this number:
- Swap index 3 with index 4: &quot;111<u>12</u>&quot; -&gt; &quot;111<u>21</u>&quot;
- Swap index 2 with index 3: &quot;11<u>12</u>1&quot; -&gt; &quot;11<u>21</u>1&quot;
- Swap index 1 with index 2: &quot;1<u>12</u>11&quot; -&gt; &quot;1<u>21</u>11&quot;
- Swap index 0 with index 1: &quot;<u>12</u>111&quot; -&gt; &quot;<u>21</u>111&quot;
</pre>

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

<pre>
<strong>Input:</strong> num = &quot;00123&quot;, k = 1
<strong>Output:</strong> 1
<strong>Explanation:</strong> The 1<sup>st</sup> smallest wonderful number is &quot;00132&quot;. To get this number:
- Swap index 3 with index 4: &quot;001<u>23</u>&quot; -&gt; &quot;001<u>32</u>&quot;
</pre>

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

<ul>
	<li><code>2 &lt;= num.length &lt;= 1000</code></li>
	<li><code>1 &lt;= k &lt;= 1000</code></li>
	<li><code>num</code> only consists of digits.</li>
</ul>
Greedy; Two Pointers; String
func getMinSwaps(num string, k int) (ans int) {
	s := []byte(num)
	for ; k > 0; k-- {
		nextPermutation(s)
	}
	d := [10][]int{}
	for i, c := range num {
		j := int(c - '0')
		d[j] = append(d[j], i)
	}
	idx := [10]int{}
	n := len(s)
	arr := make([]int, n)
	for i, c := range s {
		j := int(c - '0')
		arr[i] = d[j][idx[j]]
		idx[j]++
	}
	for i := 0; i < n; i++ {
		for j := 0; j < i; j++ {
			if arr[j] > arr[i] {
				ans++
			}
		}
	}
	return
}

func nextPermutation(nums []byte) bool {
	n := len(nums)
	i := n - 2
	for i >= 0 && nums[i] >= nums[i+1] {
		i--
	}
	if i < 0 {
		return false
	}
	j := n - 1
	for j >= 0 && nums[j] <= nums[i] {
		j--
	}
	nums[i], nums[j] = nums[j], nums[i]
	for i, j = i+1, n-1; i < j; i, j = i+1, j-1 {
		nums[i], nums[j] = nums[j], nums[i]
	}
	return true
}