0510-II173942555757-II64

Posted 2021-09-06 保护眼睛

剑指offer

• • • 剑指 Offer 42. 连续子数组的最大和
    • 剑指 Offer 10- II. 青蛙跳台阶问题
    • 剑指 Offer 05. 替换空格
    • 剑指 Offer 55 - I. 二叉树的深度
    • 剑指 Offer 17. 打印从1到最大的n位数
    • 剑指 Offer 57 - II. 和为s的连续正数序列
    • 剑指 Offer 57. 和为s的两个数字
    • 剑指 Offer 64. 求1+2+…+n
    • 剑指 Offer 39. 数组中出现次数超过一半的数字

剑指 Offer 42. 连续子数组的最大和

class Solution42 {
    public int maxSubArray(int[] nums) {
        int len = nums.length;
        int tmpRes = 0;
        int res = nums[0];

        for (int i = 0; i < len; i++) {
            tmpRes = Math.max(nums[i], tmpRes + nums[i]);
            res = Math.max(tmpRes, res);
        }

        return res;
    }
}

剑指 Offer 10- II. 青蛙跳台阶问题

class Solution10 {
    public static int numWays(int n) {
        if (n <= 2)
            return n;
        int f1 = 1;
        int f2 = 2;
        int res = 0;

        for (int i = 3; i <= n; i++) {
            res = (f1 + f2) % 1000000007;
            f1 = f2;
            f2 = res;
        }

        return res;
    }


}

剑指 Offer 05. 替换空格

class Solution05 {
    public static String replaceSpace(String s) {
        return s.replace(" ", "%20");
    }

}

剑指 Offer 55 - I. 二叉树的深度

class Solution55 {
    public int maxDepth(TreeNode root) {
        if (root == null) return 0;

        return Math.max(maxDepth(root.left), maxDepth(root.right)) + 1;
    }

    public int maxDepthDfs(TreeNode root) {
        if (root == null) return 0;
        Queue<TreeNode> queue = new LinkedList<>();
        queue.offer(root);
        int res = 0;
        while (!queue.isEmpty()) {
            res++;
            int size = queue.size();

            while (size-- > 0) {
                TreeNode cur = queue.poll();
                if (cur.left != null) queue.offer(cur.left);
                if (cur.right != null) queue.offer(cur.right);
            }
        }

        return res;
    }
}

剑指 Offer 17. 打印从1到最大的n位数

class Solution17 {
    public static int[] printNumbers(int n) {
        StringBuilder len = new StringBuilder("1");
        for (int i = 0; i < n; i++) {
            len.append(0);
        }

        int[] res = new int[Integer.parseInt(len.toString()) - 1];
        for (int i = 1; i <= Integer.parseInt(len.toString()) - 1; i++) {
            res[i - 1] = i;
        }

        return res;
    }


}

剑指 Offer 57 - II. 和为s的连续正数序列

class Solution57_II {
    public int[][] findContinuousSequence(int target) {
        List<int[]> res = new ArrayList<>();

        int left = 1;
        int right = 1;
        int sum = 0;
        while (left <= target / 2) {
            if (sum < target) {
                sum += right;
                right++;
            } else if (sum > target) {
                sum -= left;
                left++;
            } else {
                int[] tmp = new int[right - left];
                for (int i = left; i < right; i++) {
                    tmp[i - left] = i;
                }
                res.add(tmp);
                sum -= left;
                left++;
            }
        }

        return res.toArray(new int[res.size()][]);
    }
}

剑指 Offer 57. 和为s的两个数字

class Solution57 {
    public int[] twoSum(int[] nums, int target) {
        int i = 0, j = nums.length - 1;

        while (i < j) {
            int sum = nums[i] + nums[j];

            if (sum == target) {
                return new int[]{nums[i], nums[j]};
            } else if (sum > target) {
                j--;
            } else {
                i++;
            }
        }
        return new int[0];
    }
}

剑指 Offer 64. 求1+2+…+n

class Solution64 {
    public static int sumNums(int n) {
        boolean flag = n > 0 && (n += sumNums(n - 1)) == 0;
        return n;
    }

    public static int sumNums2(int n) {
        boolean flag = n <= 0 || (n += sumNums2(n - 1)) > 0;
        return n;
    }

    public static void main(String[] args) {
        System.out.println(sumNums2(5));
    }
}

剑指 Offer 39. 数组中出现次数超过一半的数字

class Solution39 {
    public int majorityElement(int[] nums) {
        Arrays.sort(nums);
        return nums[nums.length / 2];
    }
}

class Solution39_2 {
    private Map<Integer, Integer> countNums(int[] nums) {
        Map<Integer, Integer> counts = new HashMap<>();
        for (int num : nums) {
            if (!counts.containsKey(num)) {
                counts.put(num, 1);
            } else {
                counts.put(num, counts.get(num) + 1);
            }
        }
        return counts;
    }

    public int majorityElement(int[] nums) {
        Map<Integer, Integer> counts = countNums(nums);

        int len = nums.length;
        for (Map.Entry<Integer,Integer> entry : counts.entrySet()) {
            if (entry.getValue() > len / 2){
                return entry.getKey();
            }
        }
        return -1;
    }
}