24 Game
<p>You are given an integer array <code>cards</code> of length <code>4</code>. You have four cards, each containing a number in the range <code>[1, 9]</code>. You should arrange the numbers on these cards in a mathematical expression using the operators <code>[&#39;+&#39;, &#39;-&#39;, &#39;*&#39;, &#39;/&#39;]</code> and the parentheses <code>&#39;(&#39;</code> and <code>&#39;)&#39;</code> to get the value 24.</p>

<p>You are restricted with the following rules:</p>

<ul>
	<li>The division operator <code>&#39;/&#39;</code> represents real division, not integer division.

    <ul>
    	<li>For example, <code>4 / (1 - 2 / 3) = 4 / (1 / 3) = 12</code>.</li>
    </ul>
    </li>
    <li>Every operation done is between two numbers. In particular, we cannot use <code>&#39;-&#39;</code> as a unary operator.
    <ul>
    	<li>For example, if <code>cards = [1, 1, 1, 1]</code>, the expression <code>&quot;-1 - 1 - 1 - 1&quot;</code> is <strong>not allowed</strong>.</li>
    </ul>
    </li>
    <li>You cannot concatenate numbers together
    <ul>
    	<li>For example, if <code>cards = [1, 2, 1, 2]</code>, the expression <code>&quot;12 + 12&quot;</code> is not valid.</li>
    </ul>
    </li>

</ul>

<p>Return <code>true</code> if you can get such expression that evaluates to <code>24</code>, and <code>false</code> otherwise.</p>

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

<pre>
<strong>Input:</strong> cards = [4,1,8,7]
<strong>Output:</strong> true
<strong>Explanation:</strong> (8-4) * (7-1) = 24
</pre>

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

<pre>
<strong>Input:</strong> cards = [1,2,1,2]
<strong>Output:</strong> false
</pre>

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

<ul>
	<li><code>cards.length == 4</code></li>
	<li><code>1 &lt;= cards[i] &lt;= 9</code></li>
</ul>
Array; Math; Backtracking
func judgePoint24(cards []int) bool {
	ops := [4]rune{'+', '-', '*', '/'}
	nums := make([]float64, len(cards))
	for i, num := range cards {
		nums[i] = float64(num)
	}
	var dfs func([]float64) bool
	dfs = func(nums []float64) bool {
		n := len(nums)
		if n == 1 {
			return math.Abs(nums[0]-24) < 1e-6
		}
		ok := false
		for i := 0; i < n; i++ {
			for j := 0; j < n; j++ {
				if i != j {
					var nxt []float64
					for k := 0; k < n; k++ {
						if k != i && k != j {
							nxt = append(nxt, nums[k])
						}
					}
					for _, op := range ops {
						switch op {
						case '/':
							if nums[j] == 0 {
								continue
							}
							nxt = append(nxt, nums[i]/nums[j])
						case '*':
							nxt = append(nxt, nums[i]*nums[j])
						case '+':
							nxt = append(nxt, nums[i]+nums[j])
						case '-':
							nxt = append(nxt, nums[i]-nums[j])
						}
						ok = ok || dfs(nxt)
						if ok {
							return true
						}
						nxt = nxt[:len(nxt)-1]
					}
				}
			}
		}
		return ok
	}

	return dfs(nums)
}