leetcode55 Jump Game

"""
Given an array of non-negative integers, you are initially positioned at the first index of the array.
Each element in the array represents your maximum jump length at that position.
Determine if you are able to reach the last index.
Example 1:
Input: [2,3,1,1,4]
Output: true
Explanation: Jump 1 step from index 0 to 1, then 3 steps to the last index.
Example 2:
Input: [3,2,1,0,4]
Output: false
Explanation: You will always arrive at index 3 no matter what. Its maximum
             jump length is 0, which makes it impossible to reach the last index.
"""
"""
自己AC。用一个dp数组来保存能否到达该位置
"""
class Solution1:
    def canJump(self, nums):
        dp = [False]*len(nums)
        dp[0] = True
        temp2 = 0
        for i in range(len(nums)-1):
            if dp[i] == True:
                temp1 = max(i+1, temp2) #如果这里写成temp1=i+1会超时,重复赋值了
                temp2 = min(i+nums[i], len(nums)-1)
                while temp1 <= temp2:
                    dp[temp1] = True
                    temp1 += 1
        return dp[-1]
"""
解法二：从后向前遍历 O(n)解法
"""
class Solution2:
    def canJump(self, nums):
        last = len(nums)-1
        i = last - 1
        while i >= 0:
            if(nums[i] + i) >= last:
                last = i
            i -= 1
        return last <= 0

if __name__ == '__main__':
    ans = Solution2()
    nums = [2, 3, 1, 1, 4]
    print(ans.canJump(nums))