Leetcode

946. Validate Stack Sequences

by hooni posted Apr 08, 2020
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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.


class Solution {
    public boolean validateStackSequences(int[] pushed, int[] popped) {
        int loop = pushed.length;
        Stack<Integer> stack = new Stack();

        int index = 0;
        //for (int x: pushed) {
        for(int i=0; i<loop; i++){
            //stack.push(x);
            stack.push(pushed[i]);
            while (!stack.isEmpty() && index < loop && stack.peek() == popped[index]) {
                stack.pop();
                index++;
            }
        }

        return index == loop;
    }
}


class Solution {
    public boolean validateStackSequences(int[] pushed, int[] popped) {
        int loop = pushed.length;
        int[] stack = new int[loop];
        int peek = -1;
        
        int curr = 0;
        for(int i=0; i<loop; i++){
            stack[++peek] = pushed[i];
            
            while(peek > -1 && curr < loop && stack[peek] == popped[curr]){
                peek--;
                curr++;
            }
        }
        
        return curr == loop;
    }
}


[문제] https://leetcode.com/problems/validate-stack-sequences/