ValidateStackSequences946.java

/**
 * 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:
 * 0 <= pushed.length == popped.length <= 1000
 * 0 <= pushed[i], popped[i] < 1000
 * pushed is a permutation of popped.
 * pushed and popped have distinct values.
 */
public class ValidateStackSequences946 {
    public boolean validateStackSequences(int[] pushed, int[] popped) {
        int i = 0;
        int j = 0;
        int len = pushed.length;
        Stack<Integer> st = new Stack<>();
        while (i < len && j < len) {
            while (!st.isEmpty() && j < len &&  popped[j] == st.peek()) {
                j++;
                st.pop();
            }
            if (j >= len) break;
            while (i < len && pushed[i] != popped[j]) {
                st.push(pushed[i]);
                i++;
            }
            if (i < len && pushed[i] == popped[j]) {
                st.push(pushed[i]);
                i++;
            }
        }
        while (!st.isEmpty() && j < len && popped[j] == st.peek()) {
            j++;
            st.pop();
        }
        return i == len && j == len && st.isEmpty();
    }
}