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3Sum.cpp
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3Sum.cpp
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/**
* 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 ¡Ü b ¡Ü 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)
*/
// Without using hash tables
class Solution {
public:
vector<vector<int> > threeSum(vector<int> &num) {
vector<vector<int> > ret;
if (num.size() < 3) {
return ret;
}
sort(num.begin(), num.end());
int last_a = INT_MAX;
for (int i = 0; i < num.size() - 2; i++) {
int j = i + 1;
int k = num.size() - 1;
// check duplicates
if (last_a == num[i]) {
continue;
} else {
last_a = num[i];
}
int last_b = INT_MAX, last_c = INT_MAX;
while (j < k) {
int a = num[i], b = num[j], c = num[k];
// check duplicates
if (last_b == b) {
j++;
continue;
}
if (last_c == c) {
k--;
continue;
}
if (a + b + c == 0) {
ret.push_back({a, b, c});
j++;
last_b = b;
k--;
last_c = c;
} else if (a + b + c > 0) {
k--;
last_c = c;
} else {
j++;
last_b = b;
}
}
}
return ret;
}
};
// using hash table (new, cleaner version)
class Solution {
public:
vector<vector<int> > threeSum(vector<int> &num) {
vector<vector<int> > ret;
if (num.size() < 3) {
return ret;
}
int last_a = INT_MAX;
sort(num.begin(), num.end());
for (int i = 0; i < num.size() - 2; i++) {
int a = num[i];
int sum = -a;
int last_b = INT_MAX;
unordered_set<int> visited;
if (a == last_a) {
continue;
} else {
last_a = num[i];
}
for (int j = i + 1; j < num.size(); j++) {
int b = num[j];
if (b == last_b) {
continue;
}
if (visited.count(sum - b) > 0) {
ret.push_back({a, sum - b, b});
last_b = b;
}
visited.emplace(b);
}
}
return ret;
}
};
// FIXME: using hash tables (legacy)
class Solution {
/* Two sum problem */
void two_sum(vector<vector<int> > &ret, vector<int> &num, int addition,
int sum, int begin, int end, bool smaller) {
unordered_set<int> hs;
for (int i = begin; i < end; ) {
int n = num[i];
int t = sum - n;
if (hs.count(t) > 0) {
if (smaller) {
ret.push_back({addition, t, n});
} else {
ret.push_back({t, n, addition});
}
hs.erase(t);
while (num[++i] == n)
;
} else {
hs.emplace(num[i++]);
}
}
}
public:
vector<vector<int> > threeSum(vector<int> &num) {
vector<vector<int> > ret;
int nzbegin = -1; // non-zero element index beginning
if (num.size() < 3) { // error checking
return ret;
}
// sort the array
sort(num.begin(), num.end());
// do some preprocessing work
if (num[0] < 0) {
for (int i = 1; i < num.size(); i++) {
if (num[i] >= 0) {
nzbegin = i;
break;
}
}
if (nzbegin == -1 || num[num.size() - 1] == 0) {
return ret;
}
} else if (num[0] == 0 && num[1] == 0 && num[2] == 0) {
return {{0, 0, 0}};
} else {
return ret;
}
// one negative number, two positive numbers
int last_match = numeric_limits<int>::max(); // sentinel value
if (nzbegin < num.size() - 1) {
for (int i = 0; i < nzbegin; i++) {
if (num[i] == last_match) {
continue;
}
int match = -num[i];
two_sum(ret, num, num[i], match, nzbegin, num.size(), true);
last_match = num[i];
}
}
// two negative numbers, one positive number
last_match = numeric_limits<int>::max();
if (nzbegin > 1) {
for (int i = nzbegin; i < num.size(); i++) {
if (num[i] == last_match) {
continue;
}
int match = -num[i];
two_sum(ret, num, num[i], match, 0, nzbegin, false);
last_match = num[i];
}
}
// three zero but has negative numbers
if (nzbegin < num.size() - 2 && num[nzbegin + 2] == 0) {
ret.push_back({0, 0, 0});
}
return ret;
}
};