Leetcode#15 3Sum

原题地址

跟2sum、3sum、4sum、3sum closest一系列，参见这篇文章

排序+DFS+剪枝+二分查找

如果最后一个元素不二分查找会超时？？

代码：

vector<vector<int> > res;

void dfs(vector<int> &num, vector<int> ans, int pos, int left, int sum) {
  if (left == 0) {
    if (sum == 0)
      res.push_back(ans);
    return;
  }

  if (left == 1) {
    int l = pos;
    int r = num.size() - 1;
    while (l <= r) {
      int m = (l + r) / 2;
      if (num[m] + sum == 0) {
        ans.push_back(num[m]);
        res.push_back(ans);
        ans.pop_back();
        return;
      }
      else if (num[m] + sum < 0)
        l = m + 1;
      else
        r = m - 1;
    }
    return;
  }

  unordered_set<int> old;
  for (int i = pos; i < num.size(); i++) {
    if (old.find(num[i]) != old.end())
      continue;
    if (sum + left * num[i] > 0)
      break;
    if (sum + num[i] + (left - 1 ) * num[num.size() - 1] < 0)
      continue;
    old.insert(num[i]);
    ans.push_back(num[i]);
    dfs(num, ans, i + 1, left - 1, sum + num[i]);
    ans.pop_back();
  }
}

vector<vector<int> > threeSum(vector<int> &num) {
  sort(num.begin(), num.end());
  dfs(num, vector<int>(), 0, 3, 0);
  return res;
}