README.md

946. Validate Stack Sequences

Problem Description

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.

Solution

public boolean validateStackSequences(int[] pushed, int[] popped) {

    Stack<Integer> stack = new Stack<Integer>();

    int i, j;

    for (i = 0, j = 0; i < pushed.length; i++) {
        stack.push(pushed[i]);
        while (!stack.isEmpty() && j < popped.length && stack.peek() == popped[j]) {
            stack.pop();
            j++;
        }
    }
    return stack.isEmpty() || j >= popped.length;
}

Test

Compile with javac Solution.java and run with java -ea Solution.

class Solution {
    public boolean validateStackSequences(int[] pushed, int[] popped) {
        /* see previous code */
    }

    public static void main(String[] args) {
        Solution s = new Solution();
        int[] pushed = {1, 2, 3, 4, 5};
        int[] popped = {4, 5, 3, 2, 1};
        assert s.validateStackSequences(pushed, popped);

        pushed = new int[] {1, 2, 3, 4, 5};
        popped = new int[] {4, 3, 5, 1,2};
        assert !s.validateStackSequences(pushed, popped);

        pushed = new int[] {1, 2, 3, 4, 5};
        popped = new int[] {1, 2, 3, 4, 5};
        assert s.validateStackSequences(pushed, popped);

        pushed = new int[] {1, 2, 3, 4, 5};
        popped = new int[] {1, 2, 3};
        assert s.validateStackSequences(pushed, popped);

        pushed = new int[] {1, 2, 3};
        popped = new int[] {1, 2, 3, 4, 5};
        assert s.validateStackSequences(pushed, popped);
    }
}

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