946 Validate Stack Sequences
1. Question
Given two sequencespushed
andpopped
with distinct values, returntrue
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:
0 <= pushed.length == popped.length <= 1000
0 <= pushed[i], popped[i] < 1000
pushed
is a permutation ofpopped
.pushed
andpopped
have distinct values.
2. Implementation
(1) Stack
class Solution {
public boolean validateStackSequences(int[] pushed, int[] popped) {
Stack<Integer> stack = new Stack();
int i = 0;
for (int num : pushed) {
stack.push(num);
while (!stack.isEmpty() && stack.peek() == popped[i]) {
stack.pop();
++i;
}
}
return stack.isEmpty();
}
}
3. Time & Space Complexity
Stack: Time: O(n), Space: O(n)
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