977. 有序数组的平方

class Solution {
public:
    vector<int> sortedSquares(vector<int>& nums) {
        int size = nums.size();
        vector<int> result;
        for (int i = 0; i < size; i++) {
            result.push_back(nums[i] * nums[i]);
        }
        sort(result.begin(), result.end());
        return result;
    }
};

注意点：

1. vector新增一个元素是push_back()
   2.vector.begin()、vector.end()
   3.sort()函数的用法

209. 长度最小的子数组

class Solution {
public:
    int minSubArrayLen(int target, vector<int>& nums) {
        int subLength = 0;
        int sum = 0;
        int result = INT_MAX;
        int j = 0;
        int i = j;
        while (j < nums.size()) {
            sum += nums[j];
            while (sum >=target) {
                subLength = (j-i) + 1;
                result = subLength < result ? subLength:result;
                sum -= nums[i];
                i++;
            }
            j++;
        }
        return result < INT_MAX?result:0;
    }
};

注意点：

1. 双指针
   2.滑动窗口

59. 螺旋矩阵 II

class Solution {
public:
    vector<vector<int>> generateMatrix(int n) {
        int up = 0, down = n - 1, left = 0, right = n - 1;
        int index = 1;
        vector<vector<int>> result(n, vector<int>(n));
        while (index <= n * n) {
            for (int i = left; i <= right ; i++) {
                result[up][i] = index;
                index++;
            }
            up++;
            for(int i = up; i <=down;i++){
                result[i][right] = index;
                index++;
            }
            right--;
            for(int i = right;i>=left;i--){
                result[down][i] = index;
                index++;
            }
            down--;
            for(int i= down;i>=up;i--) {
                result[i][left] = index;
                index++;
            }
            left++;
        }
        return result;
    }
};

注意点：

1. vector<int> (n) 初始化一个数量为n、值为0的数组；
   2.二维数组初始化方式 vector<vector<int>> result(n, vector<int>(n))
   就直接模拟就可以了。注意边界上下左右的初始值、循环的边界值、还有循环内部边界值怎么变就可以。