Array(9) -- Container With Most Water,Insert Delete GetRandom O(1) I, II
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Container With Most Water
Given n non-negative integers a1, a2, ..., an, where each represents a point at coordinate (i, ai). n vertical lines are drawn such that the two endpoints of line i is at (i, ai) and (i, 0). Find two lines, which together with x-axis forms a container, such that the container contains the most water.
Note: You may not slant the container and n is at least 2.
这种求和求积的问题基本都是靠双指针去解决,在移动时选择较小的指针移动,以为如果想获得比现在更大的面积,只有移动小指针才有可能。
int maxArea(vector<int>& height) { int left = 0, right = height.size() - 1; int maxArea = 0; while (left < right) { maxArea = max(maxArea, min(height[left], height[right]) * (right - left)); if (height[left] < height[right]) left++; else right--; } return maxArea; }
Insert Delete GetRandom O(1)
使用vector来进行角标访问实现O(1)的random;删除与vector队尾元素互换后再进行删除,这样每次不需移动其他数。
unordered_map + vector,对于非顺序访问的数据结构,迭代器只有 itr++这种操作,不能itr = itr + 1。
开始的版本随机数使用了srand() + rand()的方法,没有ac; 发现它的expected answer默认的是不使用srand, 因此每次产生的随机数都是一样的,这里就不计较了。
class RandomizedSet {public: /** Initialize your data structure here. */ RandomizedSet() { } /** Inserts a value to the set. Returns true if the set did not already contain the specified element. */ bool insert(int val) { if(um.find(val) != um.end()) return false; else { vec.push_back(val); um[val] = vec.size() - 1; return true; } } /** Removes a value from the set. Returns true if the set contained the specified element. */ bool remove(int val) { if(um.find(val) == um.end()) return false; else { int pos = um[val]; swap(vec[pos], vec[vec.size()-1]); um[vec[pos]] = pos; vec.pop_back(); um.erase(val); return true; } } /** Get a random element from the set. */ int getRandom() { return vec[rand() % vec.size()]; } private: unordered_map<int, int> um; vector<int> vec;};
Insert Delete GetRandom O(1) - Duplicates allowed
使用一个set来记录相同数字在vector中的角标,需要注意的一点是
如果要交换的元素和尾部元素相同,则不需要进行元素交换,直接将队尾的元素pop_back()然后修改set即可
class RandomizedCollection {public: /** Initialize your data structure here. */ RandomizedCollection() { } /** Inserts a value to the collection. Returns true if the collection did not already contain the specified element. */ bool insert(int val) { bool unfound = false; if(um.find(val) == um.end()) unfound = true; vec.push_back(val); um[val].insert(vec.size() - 1); return unfound; } /** Removes a value from the collection. Returns true if the collection contained the specified element. */ bool remove(int val) { if(um.find(val) == um.end()) return false; int pos = *(um[val].begin()); //如果要交换的元素和尾部元素相同,则不需要进行元素交换,直接将队尾的元素pop_back()然后修改set即可 if(vec[pos] != vec[vec.size()-1]){ swap(vec[pos], vec[vec.size()-1]); um[vec[pos]].erase(vec.size()-1); um[vec[pos]].insert(pos); } um[val].erase(pos); vec.pop_back(); if(um[val].empty()) um.erase(val); return true; } /** Get a random element from the collection. */ int getRandom() { if(vec.empty()) return NULL; return vec[rand() % vec.size()]; }private: unordered_map<int, unordered_set<int> > um; vector<int> vec;};
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