3 Sum leetcode java

4703 ワード

タイトル:
Given an array S of n integers, are there elements a, b, c in S such that a + b + c = 0? Find all unique triplets in the array which gives the sum of zero.
Note:
  • Elements in a triplet (a,b,c) must be in non-descending order. (ie, a ≤ b ≤ c)
  • The solution set must not contain duplicate triplets.
  •     For example, given array S = {-1 0 1 2 -1 -4},
    
        A solution set is:
        (-1, 0, 1)
        (-1, -1, 2)


     3 Sum two Sum , two sum 。
    two sum : duplicate ,3 index。
    , 。
    , 0 (num.length-3), , num[i] 3sum , i+1 , 2sum 。
    3sum==0 , hashset , 。( hashset value )
    , , ,low high 。

    O(n2)

     1     
    public ArrayList> threeSum(
    int[] num) {
     2         ArrayList> res = 
    new ArrayList>();
     3         
    if(num.length<3||num == 
    null)
     4             
    return res;
     5         
     6         HashSet> hs = 
    new HashSet>();
     7         
     8         Arrays.sort(num);
     9         
    10         
    for(
    int i = 0; i <= num.length-3; i++){
    11             
    int low = i+1;
    12             
    int high = num.length-1;
    13             
    while(low//
    since they cannot be the same one, low should not equal to high
    14 
                    
    int sum = num[i]+num[low]+num[high];
    15                 
    if(sum == 0){
    16                     ArrayList unit = 
    new ArrayList();
    17                     unit.add(num[i]);
    18                     unit.add(num[low]);
    19                     unit.add(num[high]);
    20                     
    21                     
    if(!hs.contains(unit)){
    22                         hs.add(unit);
    23                         res.add(unit);
    24                     }
    25                     
    26                     low++;
    27                     high--;
    28                 }
    else 
    if(sum > 0)
    29                     high --;
    30                  
    else
    31                     low ++;
    32             }
    33         }
    34         
    35         
    return res;
    36     }


    , duplicate , , , 3 , , :
     1     
    public ArrayList> threeSum(
    int[] num) {
     2         ArrayList> res = 
    new ArrayList>();
     3         
    if(num.length<3||num == 
    null)
     4             
    return res;
     5         
     6         Arrays.sort(num);
     7         
     8         
    for(
    int i = 0; i <= num.length-3; i++){
     9             
    if(i==0||num[i]!=num[i-1]){
    //
    remove dupicate
    10 
                    
    int low = i+1;
    11                 
    int high = num.length-1;
    12                 
    while(low13                     
    int sum = num[i]+num[low]+num[high];
    14                     
    if(sum == 0){
    15                         ArrayList unit = 
    new ArrayList();
    16                         unit.add(num[i]);
    17                         unit.add(num[low]);
    18                         unit.add(num[high]);
    19                         
    20                         res.add(unit);
    21                         
    22                         low++;
    23                         high--;
    24                         
    25                         
    while(low//
    remove dupicate
    26 
                                low++;
    27                         
    while(low//
    remove dupicate
    28 
                                high--;
    29                             
    30                     }
    else 
    if(sum > 0)
    31                         high --;
    32                      
    else
    33                         low ++;
    34                 }
    35             }
    36         }
    37         
    return res;
    38     }