1441. Build an Array With Stack Operations ---------E ~

Q:

Given an array target and an integer n. In each iteration, you will read a number from  list = {1,2,3..., n}.

Build the target array using the following operations:

• Push: Read a new element from the beginning list, and push it in the array.
• Pop: delete the last element of the array.
• If the target array is already built, stop reading more elements.

You are guaranteed that the target array is strictly increasing, only containing numbers between 1 to n inclusive.

Return the operations to build the target array.

You are guaranteed that the answer is unique.

 

Example 1:

Input: target = [1,3], n = 3
Output: ["Push","Push","Pop","Push"]
Explanation: 
Read number 1 and automatically push in the array -> [1]
Read number 2 and automatically push in the array then Pop it -> [1]
Read number 3 and automatically push in the array -> [1,3]

Example 2:

Input: target = [1,2,3], n = 3
Output: ["Push","Push","Push"]

Example 3:

Input: target = [1,2], n = 4
Output: ["Push","Push"]
Explanation: You only need to read the first 2 numbers and stop.

Example 4:

Input: target = [2,3,4], n = 4
Output: ["Push","Pop","Push","Push","Push"]

 

Constraints:

• 1 <= target.length <= 100
• 1 <= target[i] <= 100
• 1 <= n <= 100
• target is strictly increasing.

A:

class Solution {
public:
    vector<string> buildArray(vector<int>& target, int n) {
        vector<string> res;
        int index = 0;
        for(int i =1;i<=n; ++i){
            if(index< target.size()){ //otherwise stop reading early
                res.push_back("Push");
                if(i < target[index] ){
                    res.push_back("Pop");
                }
                if(target[index] == i){
                    index ++;
                }
            }
        }
        return res;
    }
};

这个题目，错在， 一开始没有读完所有的例子。

导致，没有意识到early stop 的问题。   Example 3