[leetcode] Search for a Range

思路：就是二分查找，只是查找一个数的范围，思路和二分查找相似，只是在mid查找到此数后，利用right-- 或 left++ 寻找该数的右边界和左边界。

Java代码如下：

    public int[] searchRange(int[] nums, int target) {
        int[] result = {-1, -1};
        int left = 0, right = nums.length - 1;
        while (left <= right) {
            if (nums[left] == target && nums[right] == target) { //when find both left index and right index, return.
                result[0] = left;
                result[1] = right;
                return result;
            }
            int mid = (left + right)/2;
            if (nums[mid] == target) {
                
                if (nums[right] != target) {  // this if and else is very important! 
                    right--;
                }
                else {
                    left++;
                }
                
            }
            else if (nums[mid] > target) {
                right = mid - 1;
            }
            else {
                left = mid + 1;
            }
        }
        return result;
    }