https://leetcode.com/problems/wiggle-subsequence/
A sequence of numbers is called a wiggle sequence if the differences between successive numbers strictly alternate between positive and negative. The first difference (if one exists) may be either positive or negative. A sequence with fewer than two elements is trivially a wiggle sequence.
For example,
[1,7,4,9,2,5]
is a wiggle sequence because the differences(6,-3,5,-7,3)
are alternately positive and negative. In contrast,[1,4,7,2,5]
and[1,7,4,5,5]
are not wiggle sequences, the first because its first two differences are positive and the second because its last difference is zero.Given a sequence of integers, return the length of the longest subsequence that is a wiggle sequence. A subsequence is obtained by deleting some number of elements (eventually, also zero) from the original sequence, leaving the remaining elements in their original order.
Example 1:
Input: [1,7,4,9,2,5] Output: 6 Explanation: The entire sequence is a wiggle sequence.Example 2:
Input: [1,17,5,10,13,15,10,5,16,8] Output: 7 Explanation: There are several subsequences that achieve this length. One is [1,17,10,13,10,16,8].Example 3:
Input: [1,2,3,4,5,6,7,8,9] Output: 2Follow up:
Can you do it in O(n) time?
想象成折线图就很容易了
class Solution {
public:
int wiggleMaxLength(vector<int>& nums) {
if(nums.size() == 0) return nums.size();
int st = 1;
while(st < nums.size() && nums[st] == nums[st-1]) ++st;
if(st == nums.size()) return 1;
bool neg;
int result = 2;
neg = ((nums[st] - nums[st-1]) < 0);
for(int i = st+1; i < nums.size(); ++i){
if(nums[i] == nums[i-1]) continue;
if(neg == ((nums[i] - nums[i-1]) < 0)) continue;
else{
neg = !neg;
++result;
}
}
return result;
}
};