Given an array of integers, every element appears twice except for one. Find that single one.

class Solution {
public:
    int singleNumber(vector<int>& nums) {
        sort(nums.begin(), nums.end());
        for (int i =0; i<nums.size(); i++) {
            if (nums[i]!=nums[i+1]) {
                return nums[i];
            }else{
                i++;
            }
        }
        return -1;
    }
};

You are playing the following Nim Game with your friend: There is a heap of stones on the table, each time one of you take turns to remove 1 to 3 stones. The one who removes the last stone will be the winner. You will take the first turn to remove the stones.

Both of you are very clever and have optimal strategies for the game. Write a function to determine whether you can win the game given the number of stones in the heap.

For example, if there are 4 stones in the heap, then you will never win the game: no matter 1, 2, or 3 stones you remove, the last stone will always be removed by your friend.

列举1~n情况，每4个一循环.

class Solution {
public:
    bool canWinNim(int n) {
        return n%4!=0;
    }
};

class Solution {
public:
    vector<int> nextGreaterElement(vector<int>& findNums, vector<int>& nums) {
        vector<int> res;
        for(auto num1:findNums){
            for (int i = 0; i<nums.size(); i++) {
                int log = 0;
                if (nums[i]==num1) {
                    for (int j = i; j<nums.size(); j++) {
                        if (nums[j]>num1) {
                            res.push_back(nums[j]);
                            log = 1;
                            break;
                        }
                    }
                    if (log == 0) res.push_back(-1);
                    break;
                }
            }
        }
        return res;
    }
};

class Solution {
public:
    int islandPerimeter(vector<vector<int>>& grid) {
        int squNum = 0 ,log = 0;
        for (int i = 0; i<grid.size(); i++) {
            for (int j = 0; j<grid[0].size(); j++) {
                if (grid[i][j]==1) {
                    squNum++;
                    if (i!=0&&grid[i-1][j]==1) {
                        log+=2;
                    }
                    if (j!=grid[0].size()-1&&grid[i][j+1]==1) {
                        log+=2;
                    }
                }
            }
        }
        return squNum*4-log;
    }
};

利用按位异或操作^来标记不同的位置。
c++
class Solution {
public:
int hammingDistance(int x, int y) {
int tmp = x^y;
int res = 0;
while (tmp) {
if ((tmp>>1)<<1!=tmp) {
res++;
}
tmp>>=1;
}
return res;
}
};


class Solution {
public:
    vector<string> findWords(vector<string>& words) {
        set<char> row1 = {'q', 'w', 'e', 'r', 't', 'y','u', 'i', 'o', 'p'};
        set<char> row2 = {'a', 's', 'd', 'f', 'g', 'h', 'j', 'k', 'l'};
        set<char> row3 = { 'z', 'x', 'c', 'v', 'b' ,'n', 'm'};
        vector<set<char>> rows = {row1,row2,row3};
        vector<string> res;
        for (auto str: words) {
            int log = 0;
            for (int i = 0; i<rows.size(); i++) {
                if (rows[i].count((char)tolower(str[0]))>0) {
                    log = i;
                }
            }
            for (int j = 0; j<str.length(); j++) {
                if (rows[log].count((char)tolower(str[j]))==0) {
                    break;
                }
                if (j==str.length()-1) {
                    res.push_back(str);
                }
            }
        }
        return res;
    }
};