LeetCode刷题之数组专栏

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加油，不要过度焦虑(꒪ꇴ꒪(꒪ꇴ꒪ ;)

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• 二分查找：https://leetcode.cn/problems/binary-search/

/**
 * @Auther: sht2002
 * @Date: 2022-11-01 8:14
 * @Description:左闭右开时，思考为什么while(left <= right){}
 */
public class Code01 {
    
    
    public int search(int[] nums, int target) {
    
    
        int left = 0;
        int rihgt = nums.length - 1;
        while (left <= rihgt) {
    
    
            int middle = left + ((rihgt - left) >> 1);
            if (nums[middle] == target)
                return middle;
            else if (nums[middle] > target)
                rihgt = middle - 1;
            else
                left = middle + 1;
        }
        return -1;
    }
}

• 移除元素：https://leetcode.cn/problems/remove-element/

/**
 * @Auther: sht2002
 * @Date: 2022-11-01 8:35
 * @Description:快慢指针
 */
public class Code02 {
    
    
    public int removeElement(int[] nums, int val) {
    
    
        int slow = 0;
        int fast = 0;
        while (fast < nums.length) {
    
    
            if (nums[fast] == val) {
    
    
                fast++;
            } else {
    
    
                nums[slow] = nums[fast];
                slow++;
                fast++;
            }
        }
        return slow;
    }
}

• 有序数组的平方：https://leetcode.cn/problems/squares-of-a-sorted-array/

/**
 * @Auther: sht2002
 * @Date: 2022-11-01 8:46
 * @Description:明白数组值的平方后的最大值一定在数组的两端
 */
public class Code03 {
    
    
    public int[] sortedSquares(int[] nums) {
    
    
        int[] res = new int[nums.length];
        int left = 0;
        int right = nums.length - 1;
        int index = nums.length - 1;
        while (left <= right) {
    
    
            if (nums[left] * nums[left] <= nums[right] * nums[right]) {
    
    
                res[index] = nums[right] * nums[right];
                right--;
                index--;
            } else {
    
    
                res[index] = nums[left] * nums[left];
                left++;
                index--;
            }
        }
        return res;
    }
}

• 长度最小的子数组：https://leetcode.cn/problems/minimum-size-subarray-sum/

/**
 * @Auther: sht2002
 * @Date: 2022-11-01 9:14
 * @Description:滑动窗口，当fast < nums.length成立时，一定要注意slow前移，
 * 并记录满足sum < target的最小长度，所以while中会再次嵌入while，但时间复杂度为O(n)
 */
public class Code04 {
    
    
    public int minSubArrayLen(int target, int[] nums) {
    
    
        int slow = 0;
        int fast = 0;
        int sum = 0;
        int res = Integer.MAX_VALUE;
        while (fast < nums.length) {
    
    
            sum += nums[fast];
            while (sum >= target) {
    
    
                res = Math.min(res, fast - slow + 1);
                sum -= nums[slow];
                slow++;
            }
            fast++;
        }
        return (res == Integer.MAX_VALUE) ? 0 : res;
    }
}

• 螺旋矩阵：https://leetcode.cn/problems/spiral-matrix-ii/

/**
 * @Auther: sht2002
 * @Date: 2022-11-01 9:25
 * @Description:清楚明白各个变量的实际含义，遵循前闭后开的原则
 */
public class Code05 {
    
    
    public int[][] generateMatrix(int n) {
    
    
        int x = 0, y = 0;
        int i, j;
        int middle = n / 2;
        int loop = n / 2;
        int offset = 1;
        int count = 1;
        int[][] res = new int[n][n];
        while ((loop--) > 0) {
    
    
            i = x;
            j = y;
            for (; j < n - offset; j++)
                res[i][j] = count++;
            for (; i < n - offset; i++)
                res[i][j] = count++;
            for (; j > y; j--)
                res[i][j] = count++;
            for (; i > x; i--)
                res[i][j] = count++;
            x++;
            y++;
            offset++;
        }
        if (n % 2 == 1)
            res[middle][middle] = count;
        return res;
    }
}

• 搜索插入位置：https://leetcode.cn/problems/search-insert-position/

/**
 * @Auther: sht2002
 * @Date: 2022-11-01 11:55
 * @Description:思考为什么return的是right+1;
 */
public class Code06 {
    
    
    public int searchInsert(int[] nums, int target) {
    
    
        int left = 0;
        int right = nums.length - 1;
        while (left <= right) {
    
    
            int middle = left + ((right - left) >> 1);
            if (nums[middle] == target)
                return middle;
            else if (nums[middle] > target)
                right = middle - 1;
            else
                left = middle + 1;
        }
        return (right + 1);
    }
}

• 在排序数组中查找元素的第一个和最后一个位置：https://leetcode.cn/problems/find-first-and-last-position-of-element-in-sorted-array/

/**
 * @Auther: sht2002
 * @Date: 2022-11-01 12:07
 * @Description:首先用二分查找，其次要弄明白searchRange()中两个while判断条件的含义！！！
 */
public class Code07 {
    
    
      public int[] searchRange(int[] nums, int target) {
    
    
        int index = binary(nums, target);
        if (index == -1) return new int[]{
    
    -1, -1};
        int left = index;
        int right = index;
        while ((left - 1) >= 0 && nums[left - 1] == nums[index]) {
    
    
            left--;
        }
        while ((right + 1) < nums.length && nums[right + 1] == nums[index]) {
    
    
            right++;
        }
        return new int[]{
    
    left, right};
    }
    //二分查找
    private int binary(int[] nums, int target) {
    
    
        int left = 0;
        int right = nums.length - 1;
        while (left <= right) {
    
    
            int middle = left + ((right - left) >> 1);
            if (nums[middle] == target)
                return middle;
            else if (nums[middle] > target)
                right = middle - 1;
            else
                left = middle + 1;
        }
        return -1;
    }
}

• x的平方根：https://leetcode.cn/problems/sqrtx/

/**
 * @Auther: sht2002
 * @Date: 2022-11-02 5:35
 * @Description:
 */
public class Code01 {
    
    
    public int mySqrt(int x) {
    
    
        int left = 0;
        int right = x;
        int res = -1;
        while (left <= right) {
    
    
            int middle = left + ((right - left) >> 1);
            if ((long) middle * middle <= x) {
    
    
                res = middle;
                left = middle + 1;
            } else {
    
    
                right = middle - 1;
            }
        }
        return res;
    }
}

• 有效的完全平方：https://leetcode.cn/problems/valid-perfect-square/

/**
 * @Auther: sht2002
 * @Date: 2022-11-02 5:54
 * @Description:
 */
public class Code02 {
    
    
    public boolean isPerfectSquare(int num) {
    
    
        int left = 0;
        int right = num;
        while (left <= right) {
    
    
            int middle = left + ((right - left) >> 1);
            if ((long) middle * middle == num) {
    
    
                return true;
            } else if ((long) middle * middle > num) {
    
    
                right = middle - 1;
            } else {
    
    
                left = middle + 1;
            }
        }
        return false;
    }
}

• 删除有序数组中的重复项：https://leetcode.cn/problems/remove-duplicates-from-sorted-array/

/**
 * @Auther: sht2002
 * @Date: 2022-11-02 6:01
 * @Description:为了减少内存空间，可以从下标为1的地方开始，方法也是快慢指针
 */
public class Code03 {
    
    
    public int removeDuplicates(int[] nums) {
    
    
        int slow = 0;
        int fast = 0;
        int flag = Integer.MAX_VALUE;
        while (fast < nums.length) {
    
    
            if (flag != nums[fast]){
    
    
                nums[slow] = nums[fast];
                slow++;
                flag = nums[fast];
            }
            fast++;
        }
        return slow;
    }
}

• 移动零：https://leetcode.cn/problems/move-zeroes/

/**
 * @Auther: sht2002
 * @Date: 2022-11-02 6:13
 * @Description:
 */
public class Code04 {
    
    
    public void moveZeroes(int[] nums) {
    
    
        int slow = 0;
        int fast = 0;
        while (fast < nums.length) {
    
    
            if (nums[fast] != 0) {
    
    
                swap(nums, slow, fast);
                slow++;
            }
            fast++;
        }
    }
    //交换的方法千万不能使用异或运算！！
    private void swap(int[] nums, int left, int right) {
    
    
        int temp = nums[left];
        nums[left] = nums[right];
        nums[right] = temp;
    }
}