334. Increasing Triplet Subsequence

Given an unsorted array return whether an increasing subsequence of length 3 exists or not in the array.

Formally the function should:

Return true if there exists i, j, k 

such that arr[i] < arr[j] < arr[k] given 0 ≤ i < j < k ≤ n-1 else return false.

Your algorithm should run in O(n) time complexity and O(1) space complexity.

Examples:

Given 

[1, 2, 3, 4, 5]

,

return 

true

.

Given 

[5, 4, 3, 2, 1]

,

return 

false

.

遍历更新最小的数和次小的数，如果某个数大于这两个数，则有三个数递增，返回true.

class Solution {
public:
bool increasingTriplet(vector<int>& nums) {
if (nums.size() < 3) return false;
int firstMin = INT_MAX;
int secondMin = INT_MAX;
for (int i = 0; i < nums.size(); i++){
if (nums[i] <= firstMin){//<=
firstMin = nums[i];
}
else if (nums[i] <= secondMin){
secondMin = nums[i];
}
else{
return true;
}
}
return false;
}
};