Leetcode learning records

1. Sum of two numbers

Sum of two numbers     difficulty ： Simple

Given an array of integers nums And an integer target value target, Please find... In the array And is the target value the Two Integers , And return their array subscripts .

 Example  1：
 Input ：nums = [2,7,11,15], target = 9 
 Output ：[0,1] 
 explain ： because  nums[0] + nums[1] == 9 , return  [0, 1] . 

 Example  2：
 Input ：nums = [3,2,4], target = 6  Output ：[1,2] 
 Example  3：
 Input ：nums = [3,3], target = 6  Output ：[0,1] 
 
 Tips ：
2 <= nums.length <= 103
-109 <= nums[i] <= 109
-109 <= target <= 109
 There will only be one valid answer 

answer

class Solution {
    public int[] twoSum(int[] nums, int target) {
        Map<Integer, Integer> map = new HashMap<>();
        for (int i = 0; i < nums.length; i++) {
            int sub = target - nums[i];
            if (map.containsKey(sub)) {
                return new int[]{map.get(sub), i};
            }
            map.put(nums[i], i);
        }
        return new int[]{-1, -1};
    }
}

2. Addition of two numbers

Addition of two numbers     difficulty secondary

         Here are two for you Non empty The linked list of , Represents two nonnegative integers . Each of them is based on The reverse Stored in , And each node can only store One digit . Please add up the two numbers , And returns a linked list representing sum in the same form .

 Input ：l1 = [2,4,3], l2 = [5,6,4]
 Output ：[7,0,8]
 explain ：342 + 465 = 807.

answer ：

class Solution {
    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
        ListNode newHead = new ListNode(0);
        ListNode p = l1, q = l2, cur = newHead;
        int carry = 0;
        while (p != null || q != null) {
            int x = p == null ? 0 : p.val;
            int y = q == null ? 0 : q.val;
            int sum = carry + x + y;
            carry = sum / 10;
            cur.next = new ListNode(sum % 10);
            cur = cur.next;
            if (p != null) p = p.next;
            if (q != null) q = q.next;
        }
        if (carry > 0) 
            cur.next = new ListNode(carry);
        return newHead.next;
    }
}