数字之和专题

数字之和专题

1. 两数之和
   https://leetcode-cn.com/problems/two-sum/

//一遍hashmap
class Solution {
    public int[] twoSum(int[] nums, int target) {
        Map<Integer, Integer> map = new HashMap<>();
        for (int i = 0; i < nums.length; i++) {
            int complement = target - nums[i];
            if (map.containsKey(complement)) {
                return new int[] { map.get(complement), i };
            }
            map.put(nums[i], i);
        }
        throw new IllegalArgumentException("No two sum solution");
    }
}

2. 三数之和
   https://leetcode-cn.com/problems/3sum/

class Solution {
    public List<List<Integer>> threeSum(int[] nums) {
        Arrays.sort(nums);
        List<List<Integer>> res = new ArrayList<>();
        for (int i = 0; i < nums.length - 2; i++) {
            if (nums[i] > 0) break;
            if (i > 0 && nums[i] == nums[i - 1]) continue;
            int j = i + 1, k = nums.length - 1;
            while (j < k) {
                int sum = nums[i] + nums[j] + nums[k];
                if (sum < 0) while(j < k && nums[j] == nums[++j]);
                else if (sum > 0) while(j < k && nums[k] == nums[--k]);
                else {
                    res.add(new ArrayList<Integer>(Arrays.asList(nums[i], nums[j], nums[k])));
                    while (j < k && nums[j] == nums[++j]) ;
                    while (j < k && nums[k] == nums[--k]) ;
                }
            }
        }
        return res;
    }
}

3. 四数之和
   https://leetcode-cn.com/problems/4sum/

class Solution {
    public List<List<Integer>> fourSum(int[] nums, int target) {
         Arrays.sort(nums);
        List<List<Integer>> lists = new ArrayList<>();
        if (nums.length<4) return lists;
        int a, b, c, d, sum;
        for (a = 0; a <= nums.length - 4; a++) {
            if (a > 0 && nums[a] == nums[a - 1]) continue;
            for (b = a + 1; b <= nums.length - 3; b++) {
                if (b > a + 1 && nums[b] == nums[b - 1]) continue;
                c = b + 1;
                d = nums.length - 1;
                while (c < d) {
                    sum = nums[a] + nums[b] + nums[c] + nums[d];
                    if (sum > target) {
                        d--;
                    } else if (sum < target) {
                        c++;
                    } else {
                        Integer[] cache = {nums[a], nums[b], nums[c], nums[d]};
                        lists.add(Arrays.asList(cache));
                         while (c < d && nums[c] == nums[++c]) ;    //确保nums[c] 改变了
                        while (c < d && nums[d] == nums[--d]) ;   //确保nums[d] 改变了
                    }
                }
            }
        }
        return lists;
    }
}