Leetcode算法习题 第十九周

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.
Examples:
Input: [1,7,4,9,2,5]
Output: 6
The entire sequence is a wiggle sequence.
Input: [1,17,5,10,13,15,10,5,16,8]
Output: 7
There are several subsequences that achieve this length. One is [1,17,10,13,10,16,8].
Input: [1,2,3,4,5,6,7,8,9]
Output: 2

我的解答

public:
    int wiggleMaxLength(vector<int>& nums) {
        int count = 1;
        int flag = -1;// 初始化为-1，增为1，减为0 
        if(nums.size() == 0) return 0;
        for(int i = 1; i < nums.size(); i++){
            if(nums[i]>nums[i-1]){
                if(flag != 1){
                    count++; //count++满足条件：本序列增但之前是减  或者相反 
                    flag = 1;
                }
            }
            if(nums[i]< nums[i-1]){
                if(flag != 0){
                    count++; 
                    flag = 0;
                }
            }
        }
        return count;
    }
};