[Leetcode]376. 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.

思路: 假设在下标为 i 的位置已经知道了前面最长wiggle subsequence 的值， 那么对于 i 这个位置wiggle subsequence 的值要么和前面一样，要么比之前多1;

class Solution {
public:
    int wiggleMaxLength(vector<int>& nums) {
        if (nums.size() <= 1)
            return nums.size();
        int dp[nums.size()], diff = nums[1] - nums[0]; dp[0] = 1;
        int flag = (diff > 0 ? -1 : 1);
        for (int i = 1; i != nums.size(); ++i){
            diff = nums[i] - nums[i-1];
            if ((flag > 0 && diff > 0) || (flag < 0 && diff < 0) || !diff)
                dp[i] = dp[i-1];
            else {
                dp[i] = dp[i-1] + 1;
                flag = diff;
            }
        }
        return dp[nums.size() - 1];
    }
};