数组中数值和下标相等的元素

论二分法的重要性！！！

class Solution {
    public int getNumberSameAsIndex(int[] nums) {
       int n = nums.length;
        if (n == 0)
            return -1;
        int l = 0, r = n - 1;
        while (l < r) {
            int mid = l + r >> 1;
            if (nums[mid] >= mid)
                r = mid;
            if (nums[mid] < mid)
                l = mid + 1;
        }
        if (nums[l] == l)
            return l;
        else
            return -1;
    }
}