LeetCode #12 (#122、#125、#136)

122. Best Time to Buy and Sell Stock II

Say you have an array prices for which the ith element is the price of a given stock on day i.

Design an algorithm to find the maximum profit. You may complete as many transactions as you like (i.e., buy one and sell one share of the stock multiple times).

Note: You may not engage in multiple transactions at the same time (i.e., you must sell the stock before you buy again).

Example 1:

Input: [7,1,5,3,6,4]
Output: 7
Explanation: Buy on day 2 (price = 1) and sell on day 3 (price = 5), profit = 5-1 = 4.
            Then buy on day 4 (price = 3) and sell on day 5 (price = 6), profit = 6-3 = 3.

Example 2:

Input: [1,2,3,4,5]
Output: 4
Explanation: Buy on day 1 (price = 1) and sell on day 5 (price = 5), profit = 5-1 = 4.
            Note that you cannot buy on day 1, buy on day 2 and sell them later, as you are
            engaging multiple transactions at the same time. You must sell before buying again.

Example 3:

Input: [7,6,4,3,1]
Output: 0c
Explanation: In this case, no transaction is done, i.e. max profit = 0.

Constraints:

• 1 <= prices.length <= 3 * 10 ^ 4
• 0 <= prices[i] <= 10 ^ 4

//Solution
int maxProfit(int* prices, int pricesSize){
    int gain = 0;
    int last = prices[0];
    for(int i =1;i<pricesSize;i++)
    {
        if(prices[i] > last)
        {
            gain = gain +prices[i]-last;
        }
        last = prices[i];
    }
    return gain;
}

125. Valid Palindrome

Given a string, determine if it is a palindrome, considering only alphanumeric characters and ignoring cases.

Note: For the purpose of this problem, we define empty string as valid palindrome.

Example 1:

Input: "A man, a plan, a canal: Panama"
Output: true

Example 2:

Input: "race a car"
Output: false

//Solution
bool isPalindrome(char* s)
{
    if(s==NULL || !strcmp(s,"")) return true;
    for(char* p = s + strlen(s) -1; p>s;)
    if(isalnum(*s) && isalnum(*p) && tolower(*(s++)) != tolower(*(p--))) return false;            
    else if(!isalnum(*s))  s++;
    else if(!isalnum(*p))  p--;

    return true;
}

136. Single Number

Given a non-empty array of integers, every element appears twice except for one. Find that single one.

Note:

Your algorithm should have a linear runtime complexity. Could you implement it without using extra memory?

Example 1:

Input: [2,2,1]
Output: 1

Example 2:

Input: [4,1,2,1,2]
Output: 4

//Solution
//总结：一开始想着是通过一顿排序，判断前后是否与这个数相等，也是可以，后来发现通过异或运算快很多。


int singleNumber(int* nums, int numsSize){
    int res= 0;
    for(int i =0;i<numsSize;i++)
    {
        res = res ^nums[i];
    }
    return res;
}