lc209长度最小的数组

思路

• 双指针实现较低的时间复杂度
• 终止节点不断遍历，达到目标和进行窗口收缩

代码

class Solution {
    public int minSubArrayLen(int target, int[] nums) {
        int minLen = Integer.MAX_VALUE;
        int left = 0;
        int sum = 0;
        for (int right = 0; right < nums.length; right++){
            sum += nums[right];
            while (sum >= target){
                minLen = Math.min(right-left+1, minLen);
                sum -= nums[left];
                left++;
            }
        }
        return minLen == Integer.MAX_VALUE ? 0 : minLen;
    }
}

lc59螺旋矩阵II

思路

• 确定循环次数、循环不变量
• 确定四条边遍历时的下标
• 判断是否存在中心点

代码

class Solution {
    public int[][] generateMatrix(int n) {
        int loop = n / 2, val = 1;
        int[][] res = new int[n][n];
        int i,j,start = 0;
        for (int k = 1; k <= loop; k++){
            for (j = start; j < n-k; j++){
                res[start][j] = val++;
            }
            for (i = start; i < n-k; i++){
                res[i][j] = val++;
            }
            for (; j > start; j--){
                res[i][j] = val++;
            }
            for (; i > start; i--){
                res[i][j] = val++;
            }
            start++;
        }
        if (n%2 != 0){ res[n/2][n/2]=val; }
        return res;
    }
}