标签：
1，思考问题时一定要发散，基于固有的经验
2，解题思路
a，数据结构（容器）
b，基本算法（搜索，匈牙利，并查集，动态规划，网络流）
c，基本算法思想（贪心，分支）
3，本题刚开始想的是用快排后使用map，复杂度是O（n2），超时，代码如下
class Solution {
public:
vector<vector<int>> threeSum(vector<int>& nums) {
int countIn,countOut;
int length = nums.size() - 1;
int temp;
int place = 0;
vector<int> resultTemp;
vector<vector<int>> result;
map<int,int> unMap;
if(length < 2)
return result;
quickSort(nums,0,length);
for(countIn = 0 ; countIn <= length ; countIn ++){
unMap[nums[countIn]] = countIn;
if(countIn != length)
if(nums[countIn] < 0 && nums[countIn + 1] >= 0)
place = countIn;
}
for(countOut = 0 ; countOut <= place ; countOut ++){
if(countOut != 0 && nums[countOut] == nums[countOut -1])
continue;
for(countIn = length ; countIn >= place ; countIn --){
temp = 0 - (nums[countOut] + nums[countIn]);
if(unMap.find(temp) != unMap.end() && unMap[temp] > countOut && unMap[temp] < countIn){
resultTemp.push_back(nums[countOut]);
resultTemp.push_back(temp);
resultTemp.push_back(nums[countIn]);
result.push_back({nums[countOut] , temp , nums[countIn]});
}
}
}
return result;
};
void quickSort(vector<int>& nums, int low, int high){
int left,right;
int where;
int place;
left = low;
right = high;
where = 1;
place = nums[left];
if(left >= right)
return ;
while(left < right){
if(where == 1){
if(place > nums[right]){
nums[left] = nums[right];
left += 1;
where = 0;
}else{
right -= 1;
}
}else{
if(place < nums[left]){
nums[right] = nums[left];
right -= 1;
where = 1;
}else{
left += 1;
}
}
}
nums[left] = place;
quickSort(nums, low, left-1);
quickSort(nums, left+1, high);
};
};
4，后来看的网上解法，左右夹逼的方法，也是O（n2）
class Solution {
public:
vector<vector<int>> threeSum(vector<int>& nums) {
int countOut,left,right;
int length = nums.size() - 1;
int sum;
vector<int> resultTemp;
vector<vector<int>> result;
if(length < 2)
return result;
quickSort(nums,0,length);
for(countOut = 0 ; countOut <= length ; countOut ++){
if(countOut != 0 && nums[countOut] == nums[countOut -1])
continue;
left = countOut + 1;
right = length;
while(left < right){
sum = nums[countOut] + nums[left] + nums[right];
if(sum == 0){
result.push_back({nums[countOut] , nums[left] , nums[right]});
while(++left < right && nums[left] == nums[left - 1]);
while(--right > left && nums[right] == nums[right + 1]);
}else if(sum < 0){
left ++;
}else if(sum > 0){
right --;
}
}
}
return result;
};
void quickSort(vector<int>& nums, int low, int high){
int left,right;
int where;
int place;
left = low;
right = high;
where = 1;
place = nums[left];
if(left >= right)
return ;
while(left < right){
if(where == 1){
if(place > nums[right]){
nums[left] = nums[right];
left += 1;
where = 0;
}else{
right -= 1;
}
}else{
if(place < nums[left]){
nums[right] = nums[left];
right -= 1;
where = 1;
}else{
left += 1;
}
}
}
nums[left] = place;
quickSort(nums, low, left-1);
quickSort(nums, left+1, high);
};
};
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原文地址：http://www.cnblogs.com/jackes/p/5347355.html
