Split Array into Fibonacci Sequence
<p>You are given a string of digits <code>num</code>, such as <code>&quot;123456579&quot;</code>. We can split it into a Fibonacci-like sequence <code>[123, 456, 579]</code>.</p>

<p>Formally, a <strong>Fibonacci-like</strong> sequence is a list <code>f</code> of non-negative integers such that:</p>

<ul>
	<li><code>0 &lt;= f[i] &lt; 2<sup>31</sup></code>, (that is, each integer fits in a <strong>32-bit</strong> signed integer type),</li>
	<li><code>f.length &gt;= 3</code>, and</li>
	<li><code>f[i] + f[i + 1] == f[i + 2]</code> for all <code>0 &lt;= i &lt; f.length - 2</code>.</li>
</ul>

<p>Note that when splitting the string into pieces, each piece must not have extra leading zeroes, except if the piece is the number <code>0</code> itself.</p>

<p>Return any Fibonacci-like sequence split from <code>num</code>, or return <code>[]</code> if it cannot be done.</p>

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

<pre>
<strong>Input:</strong> num = &quot;1101111&quot;
<strong>Output:</strong> [11,0,11,11]
<strong>Explanation:</strong> The output [110, 1, 111] would also be accepted.
</pre>

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

<pre>
<strong>Input:</strong> num = &quot;112358130&quot;
<strong>Output:</strong> []
<strong>Explanation:</strong> The task is impossible.
</pre>

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

<pre>
<strong>Input:</strong> num = &quot;0123&quot;
<strong>Output:</strong> []
<strong>Explanation:</strong> Leading zeroes are not allowed, so &quot;01&quot;, &quot;2&quot;, &quot;3&quot; is not valid.
</pre>

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

<ul>
	<li><code>1 &lt;= num.length &lt;= 200</code></li>
	<li><code>num</code> contains only digits.</li>
</ul>
String; Backtracking
class Solution {
    private List<Integer> ans = new ArrayList<>();
    private String num;

    public List<Integer> splitIntoFibonacci(String num) {
        this.num = num;
        dfs(0);
        return ans;
    }

    private boolean dfs(int i) {
        if (i == num.length()) {
            return ans.size() >= 3;
        }
        long x = 0;
        for (int j = i; j < num.length(); ++j) {
            if (j > i && num.charAt(i) == '0') {
                break;
            }
            x = x * 10 + num.charAt(j) - '0';
            if (x > Integer.MAX_VALUE
                || (ans.size() >= 2 && x > ans.get(ans.size() - 1) + ans.get(ans.size() - 2))) {
                break;
            }
            if (ans.size() < 2 || x == ans.get(ans.size() - 1) + ans.get(ans.size() - 2)) {
                ans.add((int) x);
                if (dfs(j + 1)) {
                    return true;
                }
                ans.remove(ans.size() - 1);
            }
        }
        return false;
    }
}