现在有N个数字，其中只有一个数字出现的次数是奇数次，如何找到这个数字？

<code><span>// 示例数组</span><span>$arr</span> = <span>array</span>(<span>1</span>, <span>2</span>, <span>2</span>, <span>3</span>, <span>1</span>, <span>3</span>, <span>4</span>, <span>5</span>, <span>5</span>);

    <span>// 0与任何数异或都是其本身</span><span>$res</span> = <span>0</span>;
    <span>foreach</span> (<span>$arr</span><span>as</span><span>$value</span>) {
        <span>// 相同两个数异或结果为0</span><span>$res</span> = <span>$res</span> ^ <span>$value</span>;
    }

    var_dump(<span>$res</span>);</code>