Day01 n数之和

1、两数之和

• 思路
  • O(n^2) 循环遍历
  • O(n) 哈希表
• 题解

  class Solution {
  public int[] twoSum(int[] nums, int target) {
    int[] result = new int[2];
  
    return result;
  }
  public int[] twoSum1(int[] nums, int target) {
    int[] result = new int[2];
    for (int i = 0; i < nums.length; i++) {
       for (int j = i + 1; j < nums.length; j++) {
          if (nums[j] + nums[i] == target){
             result[0] = i;
             result[1] = j;
          }
       }
    }
    return result;
  }
  }
  

2、三数之和

• 思路
  • for循环+hash遍历
  • 双指针解法
• 题解

  class Solution {
  public List<List<Integer>> threeSum(int[] nums) {
      List<List<Integer>> ans = new ArrayList<>();
      //双指针移动、重复结果集过滤
      Arrays.sort(nums);
      for (int i = 0; i < nums.length; i++) {
          if (nums[i] > 0) return ans;
          //重复性过滤！！！注意思考
          if (i > 0 && nums[i] == nums[i - 1]) continue;
          int left = i + 1;
          int right = nums.length - 1;
          while (left < right){
              int sum = nums[i] + nums[left] + nums[right];
              if (sum > 0){
                  right --;
              }else if (sum < 0){
                  left ++;
              }else {
                  ans.add(Arrays.asList(nums[i],nums[left],nums[right]));
                  while (left < right && nums[left + 1] == nums[left]) left ++;
                  while (right > left && nums[right - 1] == nums[right]) right --;
                  right --;
                  left ++;
              }
          }
      }
      return ans;
   }
  }