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  • [LeetCode] 3 Sum, Solution


    Given an array S of n integers, are there elements abc 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)
    » Solve this problem

    [Thoughts]
    Two-pointer scan.

    [Code]
    1:    vector<vector<int> > threeSum(vector<int> &num) {  
    2: // Start typing your C/C++ solution below
    3: // DO NOT write int main() function
    4: std::sort(num.begin(), num.end());
    5: vector<vector<int> > result;
    6: int len = num.size();
    7: for(int i =0; i< len; i++)
    8: {
    9: int target = 0-num[i];
    10: int start = i+1, end =len-1;
    11: while(start<end)
    12: {
    13: if(num[start] + num[end] == target)
    14: {
    15: vector<int> solution;
    16: solution.push_back(num[i]);
    17: solution.push_back(num[start]);
    18: solution.push_back(num[end]);
    19: result.push_back(solution);
    20: start++; end--;
    21:
    while(start<end && num[start] == num[start-1]) start++;
    22:
    while(start<end && num[end] == num[end+1]) end--;
    23: }
    24: else if(num[start] + num[end] < target)
    25: {
    26: start++;
    27: }
    28: else
    29: {
    30: end--;
    31: }
    32: }
    33: if(i<len-1)
    34: {
    35:
    while(num[i] == num[i+1]) i++;
    36: }
    37: }
    38: return result;
    39: }

    [Some tricks]
    1. Line 21 and Line 22.
        filter the duplicate during two-pointer scan. For example [-2, 0, 0, 2,2], the expected output should be [-2,0,2]. If no filter here, the output will be duplicate as [-2,0,2] and [-2,0,2]
    2. Line 35
       filter the duplicate for outside iteration. For example [-2, -2, -2, 0,2].
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  • 原文地址:https://www.cnblogs.com/codingtmd/p/5078923.html
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