两种方法求解：判断出栈合法性

方法一：根据出栈顺序判断

       比当前出栈值小的，一定是递减的（必须保证入栈顺序递增）

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

        if(popped.length == 0) return true;
        int max = popped[0];

        for(int i=1; i<popped.length; i++){

            if(popped[i-1] < popped[i] && popped[i] < max)
                return false;
            max = Math.max(Math.max(popped[i-1], popped[i]), max);
        }
        return true;
    }

方法二：模拟栈进出判断

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

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

        for(int i=0, j=0; i<pushed.length; i++){

            stack.push(pushed[i]);
            while(!stack.isEmpty() && stack.peek() == popped[j]){

                stack.pop();
                j++;
            }
        }
        return stack.isEmpty();
    }