Question

Given two sequences pushed and popped with distinct values, return true if and only if this could have been the result of a sequence of push and pop operations on an initially empty stack.

Example 1:

Input: pushed = [1,2,3,4,5], popped = [4,5,3,2,1] Output: true Explanation: We might do the following sequence: push(1), push(2), push(3), push(4), pop() -> 4, push(5), pop() -> 5, pop() -> 3, pop() -> 2, pop() -> 1

Example 2:

Input: pushed = [1,2,3,4,5], popped = [4,3,5,1,2] Output: false Explanation: 1 cannot be popped before 2.

Note:

1. 0 <= pushed.length == popped.length <= 1000
2. 0 <= pushed[i], popped[i] < 1000
3. pushed is a permutation of popped.
4. pushed and popped have distinct values.

Difficulty:Medium

Category:Stack

Analyze

Solution

class Solution {
 public:
  bool validateStackSequences(vector<int>& pushed, vector<int>& popped) {
    stack<int> s;
    auto it = popped.begin();
    for (int& e : pushed) {
      s.push(e);
      while (!s.empty() && s.top() == *it) {
        s.pop();
        ++it;
      }
    }
    return it == popped.end();
  }
};

Solution 2: Stack

class Solution {
 public:
  vector<int> nextGreaterElements(vector<int>& nums) {
    int n = nums.size();
    vector<int> r(n, -1);
    stack<int> s;
    for (int i = 0; i < 2 * n; ++i) {
      // Deal with the element;
      int temp = nums[i % n];
      while (!s.empty() && temp > nums[s.top()]) {
        r[s.top()] = temp;
        s.pop();
      }
      // save the element;
      if (i < n) s.push(i);
    }
    return r;
  }
};