Merge Intervals

https://leetcode.com/problems/merge-intervals/
合并区间，当数组之间存在交集的时候，讲数组合并。（这里的数组类都定义好了，就是两个int的区间）

/**
 * Definition for an interval.
 * class Interval {
 *     public $start = 0;
 *     public $end = 0;
 *     function __construct(int $start = 0, int $end = 0) {
 *         $this->start = $start;
 *         $this->end = $end;
 *     }
 * }
 */
    /**
     * @param Interval[] $intervals
     * @return Interval[]
     */
    function merge($intervals) {
        if(!$intervals){
            return [];
        }
        usort($intervals,function($intervalsa,$intervalsb){
            return $intervalsa->start <= $intervalsb->start ? -1 : 1;
        });

        $res[] = $intervals[0];
        for($i = 1; $i<count($intervals);$i++){
            if($res[count($res) - 1]->end < $intervals[$i]->start){
                $res[] = $intervals[$i];
            }else{
                $res[count($res) - 1]->end = max($res[count($res) - 1]->end,$intervals[$i]->end);
            }
        }
        return $res;
    }