Wiggle Subsequence

Desicription

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:

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Input: [1,7,4,9,2,5]
Output: 6
Explanation: The entire sequence is a wiggle sequence.

Example 2:

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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:

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Input: [1,2,3,4,5,6,7,8,9]
Output: 2

Follow up:

Can you do it in O(n) time?

Solution

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class Solution {
public:
int wiggleMaxLength(std::vector<int>& nums) {

if(nums.empty()) {
return 0;
}

bool isPositive_1 = false;
auto stack_1 = std::stack<int>();
for(int i = 0; i < nums.size(); i++) {
if(i == 0) {
stack_1.push(nums[i]);
} else {
if(isPositive_1) {
if(nums[i] > stack_1.top()) {
stack_1.pop();
stack_1.push(nums[i]);
} else if(nums[i] < stack_1.top()) {
stack_1.push(nums[i]);
isPositive_1 = false;
}
} else {
if(nums[i] > stack_1.top()) {
stack_1.push(nums[i]);
isPositive_1 = true;
} else if(nums[i] < stack_1.top()) {
stack_1.pop();
stack_1.push(nums[i]);
}
}
}
}

bool isPositive_2 = true;
auto stack_2 = std::stack<int>();
for(int i = 0; i < nums.size(); i++) {
if(i == 0) {
stack_2.push(nums[i]);
} else {
if(isPositive_2) {
if(nums[i] > stack_2.top()) {
stack_2.pop();
stack_2.push(nums[i]);
} else if(nums[i] < stack_2.top()) {
stack_2.push(nums[i]);
isPositive_2 = false;
}
} else {
if(nums[i] > stack_2.top()) {
stack_2.push(nums[i]);
isPositive_2 = true;
} else if(nums[i] < stack_2.top()) {
stack_2.pop();
stack_2.push(nums[i]);
}
}
}
}

return std::max(stack_1.size(), stack_2.size());
}
};