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• 400 Nth Digit
• 401 Binary Watch
• 402 Remove K Digits
• 403 Frog Jump

### leetcode 400. Nth Digit

Find the _n_th digit of the infinite integer sequence 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, ...

Note: n is positive and will fit within the range of a 32-bit signed integer (n < 231).

Example 1:

Example 2:

### leetcode 401. Binary Watch

A binary watch has 4 LEDs on the top which represent the hours (0-11), and the 6 LEDs on the bottom represent the minutes (0-59).

Each LED represents a zero or one, with the least significant bit on the right. For example, the above binary watch reads "3:25".

Given a non-negative integer n which represents the number of LEDs that are currently on, return all possible times the watch could represent.

Example:

Note:

• The order of output does not matter.
• The hour must not contain a leading zero, for example "01:00" is not valid, it should be "1:00".
• The minute must be consist of two digits and may contain a leading zero, for example "10:2" is not valid, it should be "10:02".

### leetcode 402. Remove K Digits

Given a non-negative integer num represented as a string, remove k digits from the number so that the new number is the smallest possible.

Note:

• The length of num is less than 10002 and will be ≥ k.
• The given num does not contain any leading zero.

Example 1:

Example 2:

Example 3:

### leetcode 403. Frog Jump

A frog is crossing a river. The river is divided into x units and at each unit there may or may not exist a stone. The frog can jump on a stone, but it must not jump into the water.

Given a list of stones' positions (in units) in sorted ascending order, determine if the frog is able to cross the river by landing on the last stone. Initially, the frog is on the first stone and assume the first jump must be 1 unit.

If the frog's last jump was k units, then its next jump must be either k - 1, k, or k + 1 units. Note that the frog can only jump in the forward direction.

Note:

• The number of stones is ≥ 2 and is < 1,100.
• Each stone's position will be a non-negative integer < 231.
• The first stone's position is always 0.

Example 1:

Example 2:

### 另一种写法

dp[j][i] 从i可以到达j，因此，对于点 j，我们只需要查看可以从哪个地方跳转过来（这里假设为i），然后查看其跳跃的距离$$step = stones[j] - stones[i]$$ , 则下一次的跳的距离为$$step + 1, step, step - 1$$ ，然后查看下一个点_id存不存在（用Hash），存在将dp[_id][j] 设置为可达 ,若$$id==n-1$$，说明到达了对岸。这样复杂度为O(n^2)

• 400 Nth Digit
• 401 Binary Watch
• 402 Remove K Digits
• 403 Frog Jump