Leetcode [724, 747,66] Array专题 (1)_for (int item: a1) {-优快云博客

博客围绕数组操作展开，介绍了三个算法问题。包括找出数组的“枢轴”索引，判断数组中最大元素是否至少是其他元素两倍并返回其索引，以及对表示非负整数的数组加一。还给出了各问题的复杂度分析，时间复杂度多为O(N)，空间复杂度多为O(1)。

Operations in Array

// "static void main" must be defined in a public class.
public class Main {
    public static void main(String[] args) {
        // 1. Initialize
        int[] a0 = new int[5];
        int[] a1 = {1, 2, 3};
        // 2. Get Length
        System.out.println("The size of a1 is: " + a1.length);
        // 3. Access Element
        System.out.println("The first element is: " + a1[0]);
        // 4. Iterate all Elements
        System.out.print("[Version 1] The contents of a1 are:");
        for (int i = 0; i < a1.length; ++i) {
            System.out.print(" " + a1[i]);
        }
        System.out.println();
        System.out.print("[Version 2] The contents of a1 are:");
        for (int item: a1) {
            System.out.print(" " + item);
        }
        System.out.println();
        // 5. Modify Element
        a1[0] = 4;
        // 6. Sort
        Arrays.sort(a1);
    }
}

// "static void main" must be defined in a public class.
public class Main {
    public static void main(String[] args) {
        // 1. initialize
        List<Integer> v0 = new ArrayList<>();
        List<Integer> v1;                           // v1 == null
        // 2. cast an array to a vector
        Integer[] a = {0, 1, 2, 3, 4};
        v1 = new ArrayList<>(Arrays.asList(a));
        // 3. make a copy
        List<Integer> v2 = v1;                      // another reference to v1
        List<Integer> v3 = new ArrayList<>(v1);     // make an actual copy of v1
        // 3. get length
        System.out.println("The size of v1 is: " + v1.size());
        // 4. access element
        System.out.println("The first element in v1 is: " + v1.get(0));
        // 5. iterate the vector
        System.out.print("[Version 1] The contents of v1 are:");
        for (int i = 0; i < v1.size(); ++i) {
            System.out.print(" " + v1.get(i));
        }
        System.out.println();
        System.out.print("[Version 2] The contents of v1 are:");
        for (int item : v1) {
            System.out.print(" " + item);
        }
        System.out.println();
        // 6. modify element
        v2.set(0, 5);       // modify v2 will actually modify v1
        System.out.println("The first element in v1 is: " + v1.get(0));
        v3.set(0, -1);
        System.out.println("The first element in v1 is: " + v1.get(0));
        // 7. sort
        Collections.sort(v1);
        // 8. add new element at the end of the vector
        v1.add(-1);
        v1.add(1, 6);
        // 9. delete the last element
        v1.remove(v1.size() - 1);
    }
}

724 Find Pivot Index

Given an array of integers nums, write a method that returns the "pivot" index of this array.

We define the pivot index as the index where the sum of the numbers to the left of the index is equal to the sum of the numbers to the right of the index.

If no such index exists, we should return -1. If there are multiple pivot indexes, you should return the left-most pivot index.

Example 1:

Input: 
nums = [1, 7, 3, 6, 5, 6]
Output: 3
Explanation: 
The sum of the numbers to the left of index 3 (nums[3] = 6) is equal to the sum of numbers to the right of index 3.
Also, 3 is the first index where this occurs.

Example 2:

Input: 
nums = [1, 2, 3]
Output: -1
Explanation: 
There is no index that satisfies the conditions in the problem statement.

Note:

The length of nums will be in the range [0, 10000].
Each element nums[i] will be an integer in the range [-1000, 1000].

class Solution {
    public int pivotIndex(int[] nums) {
        int len = nums.length;
        for(int i=1;i<len-1;i++){
            int leftSum = 0;
            int rightSum = 0;
            for(int j=0; j<i;j++){
                leftSum+=nums[j];
            }
            for(int k=i+1;k<len;k++){
                rightSum+=nums[k];        
            }
            if(leftSum == rightSum){
                    return i;
            }
        }
        return -1;
    }
}

//更简洁的版本
class Solution {
    public int pivotIndex(int[] nums) {
        int sum = 0, leftsum = 0;
        for (int x: nums) sum += x;
        for (int i = 0; i < nums.length; ++i) {
            if (leftsum == sum - leftsum - nums[i]) return i;
            leftsum += nums[i];
        }
        return -1;
    }
}

Complexity Analysis

Time Complexity: O(N), where NN is the length of nums.
Space Complexity: O(1), the space used by leftsum and S.

747 Largest Number At Least Twice of Others

In a given integer array nums, there is always exactly one largest element.

Find whether the largest element in the array is at least twice as much as every other number in the array.

If it is, return the index of the largest element, otherwise return -1.

Example 1:

Input: nums = [3, 6, 1, 0]
Output: 1
Explanation: 6 is the largest integer, and for every other number in the array x,
6 is more than twice as big as x.  The index of value 6 is 1, so we return 1.

Example 2:

Input: nums = [1, 2, 3, 4]
Output: -1
Explanation: 4 isn't at least as big as twice the value of 3, so we return -1.

Note:

nums will have a length in the range [1, 50].
Every nums[i] will be an integer in the range [0, 99].

class Solution {
    public int dominantIndex(int[] nums) {
        int max = -1, index = -1, second = -1;
        for (int i = 0; i < nums.length; i++) {
            if (nums[i] > max) {
                second = max;
                max = nums[i];
                index = i;
            } else if (nums[i] > second)
                second = nums[i];
        }
        if(second*2 <=max)
            return index;
        else
            return -1;
    }
}

Complexity Analysis

Time Complexity: O(N) where N is the length of nums.
Space Complexity: O(1), the space used by our int variables.

66 Plus One

Given a non-empty array of digits representing a non-negative integer, plus one to the integer.

The digits are stored such that the most significant digit is at the head of the list, and each element in the array contain a single digit.

You may assume the integer does not contain any leading zero, except the number 0 itself.

Example 1:

Input: [1,2,3]
Output: [1,2,4]
Explanation: The array represents the integer 123.

Example 2:

Input: [4,3,2,1]
Output: [4,3,2,2]
Explanation: The array represents the integer 4321.

class Solution {
    public int[] plusOne(int[] digits) {
    for (int i = digits.length - 1; i >=0; i--) {
        if (digits[i] != 9) {
            digits[i]++;
            break;
        } else {
            digits[i] = 0;
        }
    }
    if (digits[0] == 0) {
        int[] res = new int[digits.length+1];
        res[0] = 1;
        return res;
    }
    return digits;
}
}

Complexity Analysis