56.合并区间

排序，然后逐一合并

时间复杂度： O(NlogN)
空间复杂的： O(logN)

func merge(intervals [][]int) [][]int {
    sort.Slice(intervals, func(i, j int) bool {
        if intervals[i][0] == intervals[j][0] {
            return intervals[i][1] < intervals[j][1]
        } else {
            return intervals[i][0] < intervals[j][0]
        }
    })

    ans := [][]int{}
    
    l, r := intervals[0][0], intervals[0][1]

    for i := 1; i < len(intervals); i++ {
        if intervals[i][0] <= r {
            if (intervals[i][1] > r) {
                r = intervals[i][1]
            }
            continue
        }

        ans = append(ans, []int{l, r})
        l = intervals[i][0]
        r = intervals[i][1]
    }

    ans = append(ans, []int{l, r})
    
    return ans
}