StoneWall

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problem

solution

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/// you can use includes, for example:
// #include <algorithm>

// you can write to stdout for debugging purposes, e.g.
// cout << "this is a debug message" << endl;
#include<stack>
int solution(vector<int> &H) {
    // write your code in C++14 (g++ 6.2.0)
    
    // monotonic stack
    stack<int> sta;
    int n = H.size(), block = 0;
    //  8   8   5   7   9   8   7   4   8
    //
    for(int i=0;i<n;++i){
        while(!sta.empty() && sta.top() > H[i] ){
            
            sta.pop();
        }
        if(sta.empty() || sta.top() < H[i] ) {
            sta.push(H[i]);
            block++;
        }
    }
    return block ;
}

analysis

  • time complexity O(n)
  • space complexity O(n)